SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
$ 352.6
|
$ 823.2
|
$ 978.3
|
$ 312.8
|
Accounts receivable, net |
94.0
|
226.7
|
Ìý
|
Ìý
|
Inventories |
317.4
|
181.1
|
Ìý
|
Ìý
|
Derivative assets |
45.8
|
91.5
|
Ìý
|
Ìý
|
Income tax receivable, current |
58.6
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
29.5
|
39.8
|
Ìý
|
Ìý
|
Total current assets |
897.9
|
1,479.6
|
Ìý
|
Ìý
|
Non-Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Property, plant and equipment, net |
1,929.0
|
1,286.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
62.7
|
121.3
|
Ìý
|
Ìý
|
Deferred income taxes |
459.5
|
464.8
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
154.7
|
177.9
|
Ìý
|
Ìý
|
TOTAL ASSETS |
3,503.8
|
3,529.6
|
2,953.4
|
Ìý
|
Current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
193.2
|
186.8
|
Ìý
|
Ìý
|
Accrued liabilities |
126.3
|
158.9
|
Ìý
|
Ìý
|
State and local taxes payable |
37.9
|
35.5
|
Ìý
|
Ìý
|
Other current liabilities |
52.0
|
87.0
|
Ìý
|
Ìý
|
Total current liabilities |
409.4
|
468.2
|
Ìý
|
Ìý
|
Non-current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Long-term debt |
2,113.8
|
2,092.9
|
Ìý
|
Ìý
|
Pension and OPEB liabilities |
311.5
|
248.7
|
Ìý
|
Ìý
|
Environmental and mine closure obligations |
164.9
|
172.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current liabilities |
146.3
|
123.6
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
3,145.9
|
3,105.4
|
Ìý
|
Ìý
|
Equity: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL EQUITY |
357.9
|
424.2
|
(444.1)
|
(1,330.5)
|
TOTAL LIABILITIES AND EQUITY |
3,503.8
|
3,529.6
|
Ìý
|
Ìý
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.0
|
0.0
|
0.0
|
0.0
|
Accounts receivable, net |
(4.2)
|
(4.1)
|
Ìý
|
Ìý
|
Inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Total current assets |
(4.2)
|
(4.1)
|
Ìý
|
Ìý
|
Non-Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Property, plant and equipment, net |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Deferred income taxes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
(1,868.3)
|
(1,313.1)
|
Ìý
|
Ìý
|
Long-term intercompany notes |
(121.3)
|
(121.3)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
(1,993.8)
|
(1,438.5)
|
Ìý
|
Ìý
|
Current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
(4.2)
|
(4.1)
|
Ìý
|
Ìý
|
Accrued liabilities |
0.0
|
0.0
|
Ìý
|
Ìý
|
State and local taxes payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
0.0
|
0.0
|
Ìý
|
Ìý
|
Total current liabilities |
(4.2)
|
(4.1)
|
Ìý
|
Ìý
|
Non-current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Long-term debt |
0.0
|
0.0
|
Ìý
|
Ìý
|
Pension and OPEB liabilities |
0.0
|
0.0
|
Ìý
|
Ìý
|
Environmental and mine closure obligations |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
(121.3)
|
(121.3)
|
Ìý
|
Ìý
|
Other non-current liabilities |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(125.5)
|
(125.4)
|
Ìý
|
Ìý
|
Equity: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL EQUITY |
(1,868.3)
|
(1,313.1)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
(1,993.8)
|
(1,438.5)
|
Ìý
|
Ìý
|
Parent [Member] | Reportable Legal Entities [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
349.7
|
819.8
|
948.9
|
283.4
|
Accounts receivable, net |
4.9
|
9.2
|
Ìý
|
Ìý
|
Inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
0.1
|
Ìý
|
Ìý
|
Income tax receivable, current |
58.6
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
9.1
|
10.0
|
Ìý
|
Ìý
|
Total current assets |
422.3
|
956.4
|
Ìý
|
Ìý
|
Non-Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Property, plant and equipment, net |
11.2
|
13.3
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
58.6
|
117.2
|
Ìý
|
Ìý
|
Deferred income taxes |
458.3
|
463.6
|
Ìý
|
Ìý
|
Investment in subsidiaries |
1,821.1
|
1,262.3
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
15.1
|
8.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,786.6
|
2,820.8
|
Ìý
|
Ìý
|
Current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
5.7
|
5.3
|
Ìý
|
Ìý
|
Accrued liabilities |
80.7
|
92.7
|
Ìý
|
Ìý
|
State and local taxes payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
6.0
|
4.8
|
Ìý
|
Ìý
|
Total current liabilities |
92.4
|
102.8
|
Ìý
|
Ìý
|
Non-current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Long-term debt |
2,113.8
|
2,092.9
|
Ìý
|
Ìý
|
Pension and OPEB liabilities |
80.5
|
64.3
|
Ìý
|
Ìý
|
Environmental and mine closure obligations |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
121.3
|
121.3
|
Ìý
|
Ìý
|
Other non-current liabilities |
20.7
|
15.3
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
2,428.7
|
2,396.6
|
Ìý
|
Ìý
|
Equity: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL EQUITY |
357.9
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
2,786.6
|
2,820.8
|
Ìý
|
Ìý
|
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.1
|
0.7
|
2.1
|
2.5
|
Accounts receivable, net |
93.0
|
221.3
|
Ìý
|
Ìý
|
Inventories |
317.4
|
181.1
|
Ìý
|
Ìý
|
Derivative assets |
45.8
|
91.4
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
13.0
|
16.9
|
Ìý
|
Ìý
|
Total current assets |
469.3
|
511.4
|
Ìý
|
Ìý
|
Non-Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Property, plant and equipment, net |
1,867.1
|
1,221.9
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
4.1
|
4.1
|
Ìý
|
Ìý
|
Deferred income taxes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
47.2
|
50.8
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
121.4
|
153.8
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,509.1
|
1,942.0
|
Ìý
|
Ìý
|
Current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
187.5
|
181.4
|
Ìý
|
Ìý
|
Accrued liabilities |
45.5
|
66.1
|
Ìý
|
Ìý
|
State and local taxes payable |
37.9
|
35.4
|
Ìý
|
Ìý
|
Other current liabilities |
38.6
|
74.1
|
Ìý
|
Ìý
|
Total current liabilities |
309.5
|
357.0
|
Ìý
|
Ìý
|
Non-current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Long-term debt |
0.0
|
0.0
|
Ìý
|
Ìý
|
Pension and OPEB liabilities |
496.9
|
414.4
|
Ìý
|
Ìý
|
Environmental and mine closure obligations |
145.6
|
152.1
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current liabilities |
120.3
|
99.5
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
1,072.3
|
1,023.0
|
Ìý
|
Ìý
|
Equity: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL EQUITY |
1,436.8
|
919.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
2,509.1
|
1,942.0
|
Ìý
|
Ìý
|
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
2.8
|
2.7
|
$ 27.3
|
$ 26.9
|
Accounts receivable, net |
0.3
|
0.3
|
Ìý
|
Ìý
|
Inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
7.4
|
12.9
|
Ìý
|
Ìý
|
Total current assets |
10.5
|
15.9
|
Ìý
|
Ìý
|
Non-Current Assets: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Property, plant and equipment, net |
50.7
|
50.8
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Deferred income taxes |
1.2
|
1.2
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
121.3
|
121.3
|
Ìý
|
Ìý
|
Other non-current assets |
18.2
|
16.1
|
Ìý
|
Ìý
|
TOTAL ASSETS |
201.9
|
205.3
|
Ìý
|
Ìý
|
Current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
4.2
|
4.2
|
Ìý
|
Ìý
|
Accrued liabilities |
0.1
|
0.1
|
Ìý
|
Ìý
|
State and local taxes payable |
0.0
|
0.1
|
Ìý
|
Ìý
|
Other current liabilities |
7.4
|
8.1
|
Ìý
|
Ìý
|
Total current liabilities |
11.7
|
12.5
|
Ìý
|
Ìý
|
Non-current liabilities: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Long-term debt |
0.0
|
0.0
|
Ìý
|
Ìý
|
Pension and OPEB liabilities |
(265.9)
|
(230.0)
|
Ìý
|
Ìý
|
Environmental and mine closure obligations |
19.3
|
19.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current liabilities |
5.3
|
8.8
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(229.6)
|
(188.8)
|
Ìý
|
Ìý
|
Equity: |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL EQUITY |
431.5
|
394.1
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
$ 201.9
|
$ 205.3
|
Ìý
|
Ìý
|