SHAREHOLDERS' EQUITY - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions |
3 Months Ended | 9 Months Ended | |||||||
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Mar. 13, 2020 |
Dec. 31, 2020 |
Mar. 31, 2020 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Sep. 30, 2020 |
Sep. 30, 2019 |
Apr. 24, 2019 |
Nov. 26, 2018 |
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Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Common shares, par value | Ìý | Ìý | Ìý | Ìý | Ìý | $ 0.125 | Ìý | Ìý | Ìý |
Fair value of Cliffs common shares issued for AK Steel outstanding common stock | $ 617.6 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stock Repurchase Program, Authorized Amount | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | $ 100.0 | $ 200.0 |
Stock Repurchased During Period, Shares | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 24,400,000 | Ìý | Ìý |
Payments for Repurchase of common shares | Ìý | Ìý | Ìý | Ìý | Ìý | $ 0.0 | $ 252.9 | Ìý | Ìý |
Forecast [Member] | ArcelorMittal USA [Member] | Series B Participating Redeemable Preferred Stock [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Shares of Cliffs common shares issued to AK Steel stockholders | Ìý | 583,273 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred Class A [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred Stock, Shares Authorized | Ìý | Ìý | Ìý | Ìý | Ìý | 3,000,000 | Ìý | Ìý | Ìý |
Preferred Stock, Shares Outstanding | Ìý | Ìý | Ìý | Ìý | Ìý | 0 | Ìý | Ìý | Ìý |
Preferred Stock, Shares Issued | Ìý | Ìý | Ìý | Ìý | Ìý | 0 | Ìý | Ìý | Ìý |
Preferred Class B [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Preferred Stock, Shares Authorized | Ìý | Ìý | Ìý | Ìý | Ìý | 4,000,000 | Ìý | Ìý | Ìý |
Preferred Stock, Shares Outstanding | Ìý | Ìý | Ìý | Ìý | Ìý | 0 | Ìý | Ìý | Ìý |
Preferred Stock, Shares Issued | Ìý | Ìý | Ìý | Ìý | Ìý | 0 | Ìý | Ìý | Ìý |
Common Stock [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Class of Stock [Line Items] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stock Issued During Period, Shares, Acquisitions | 126,800,000 | Ìý | 126,800,000 | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
Stock Repurchased During Period, Shares | Ìý | Ìý | Ìý | 12,900,000 | 11,500,000 | Ìý | Ìý | Ìý | Ìý |
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- Definition Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Number of shares of equity interests issued or issuable to acquire entity. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available.
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- Definition Face amount or stated value per share of common stock. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The cash outflow to reacquire common stock during the period. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Number of shares of stock issued during the period pursuant to acquisitions. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition Amount of stock repurchase plan authorized. No definition available.
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- Definition Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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