Cleveland-Cliffs Inc. Annual report pursuant to Section 13 and 15(d)

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Accumulated Other Comprehensive Income (Loss) Changes in Equity (Details)

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Accumulated Other Comprehensive Income (Loss) Changes in Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning of Period	$ (319)	$ (284)	$ (39)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	160	(59)	(41)
Net loss (gain) reclassified from accumulated other comprehensive loss	26	24	(204)
Accumulated Other Comprehensive Income (Loss), Net of Tax, End of Period	(133)	(319)	(284)
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning of Period	(316)	(281)	264
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	163	(57)	(43)
Net loss (gain) reclassified from accumulated other comprehensive loss	18	22	26
Accumulated Other Comprehensive Income (Loss), Net of Tax, End of Period	(135)	(316)	(281)
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning of Period	0	0	225
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	3	0	3
Net loss (gain) reclassified from accumulated other comprehensive loss	0	0	(228)
Accumulated Other Comprehensive Income (Loss), Net of Tax, End of Period	3	0	0
Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Beginning of Period	(3)	(3)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(6)	(2)	(1)
Net loss (gain) reclassified from accumulated other comprehensive loss	8	2	(2)
Accumulated Other Comprehensive Income (Loss), Net of Tax, End of Period	$ (1)	$ (3)	$ (3)

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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