Cleveland-Cliffs (CLF) 10-K 26 Feb 21 2020 FY Annual report Financial data

Cover

Cover - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 24, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	1-8944
Entity Registrant Name	CLEVELAND-CLIFFS INC.
Entity Incorporation, State or Country Code	OH
Entity Tax Identification Number	34-1464672
Entity Address, Address Line One	200 Public Square,
Entity Address, City or Town	Cleveland,
Entity Address, State or Province	OH
Entity Address, Postal Zip Code	44114-2315
City Area Code	216
Local Phone Number	694-5700
Title of 12(b) Security	Common Shares, par value $0.125 per share
Trading Symbol	CLF
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 2,171,029,299
Entity Common Stock, Shares Outstanding		498,885,558
Entity Central Index Key	0000764065
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Documents Incorporated by Reference [Text Block]	Portions of the registrant’s proxy statement for its 2021 annual meeting of shareholders are incorporated by reference into Part III.

Statements Of Condensed Consoli

Statements Of Condensed Consolidated Financial Position - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 112	$ 353
Accounts receivable, net	1,169	94
Inventories	3,828	317
Income tax receivable, current	24	59
Other current assets	165	75
Total current assets	5,298	898
Non-current assets:
Property, plant and equipment, net	8,743	1,929
Goodwill	1,406	2
Deferred income taxes	537	460
Other non-current assets	787	215
TOTAL ASSETS	16,771	3,504
Current liabilities:
Accounts payable	1,575	193
Accrued employment costs	460	67
State and local taxes	147	38
Pension and OPEB liabilities, current	151	4
Other current liabilities	596	107
Total current liabilities	2,929	409
Non-current liabilities:
Long-term debt	5,390	2,114
Pension and OPEB liabilities, non-current	4,113	312
Other non-current liabilities	1,260	311
TOTAL LIABILITIES	13,692	3,146
Commitments and contingencies (See Note 21)
Series B Participating Redeemable Preferred Stock - no par value. Authorized, Issued and Outstanding - 583,273	738	0
Equity:
Common Shares - par value $0.125 per share, Authorized - 600,000,000 shares (2019 - 600,000,000 shares); Issued - 506,832,537 shares (2019 - 301,886,794) shares); Outstanding - 477,517,372 shares (2019 - 270,084,005) shares)	63	37
Capital in excess of par value of shares	5,431	3,873
Retained deficit	(2,989)	(2,842)
Cost of 29,315,165 common shares in treasury (2019 - 31,802,789 shares)	(354)	(391)
Accumulated other comprehensive loss	(133)	(319)
Total Cliffs shareholders' equity	2,018	358
Noncontrolling interest	323	0
TOTAL EQUITY	2,341	358
TOTAL LIABILITIES, REDEEMABLE PREFERRED STOCK AND EQUITY	$ 16,771	$ 3,504

Statements Of Condensed Conso_2

Statements Of Condensed Consolidated Financial Position (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Temporary Equity, Par or Stated Value Per Share	$ 0
Temporary Equity, Shares Authorized	583,273
Temporary Equity, Shares Issued	583,273
Temporary Equity, Shares Outstanding	583,273
Common shares, par value	$ 0.125
Common shares, authorized (in shares)	600,000,000	600,000,000
Common shares, issued (in shares)	506,832,537	301,886,794
Common shares, outstanding (in shares)	477,517,372	270,084,005
Common shares in treasury	29,315,165	31,802,789

Statements Of Condensed Conso_3

Statements Of Condensed Consolidated Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenues	$ 5,319	$ 1,990	$ 2,332
Realization of Deferred Revenue	35	0	0
Operating Costs:
Cost of goods sold	(5,102)	(1,414)	(1,523)
Selling, general and administrative expenses	(244)	(113)	(113)
Acquisition-related costs	(90)	(7)	0
Miscellaneous – net	(60)	(27)	(23)
Total operating costs	(5,496)	(1,561)	(1,659)
Operating income (loss)	(142)	429	673
Other income (expense):
Interest expense, net	(238)	(101)	(119)
Gain (loss) on extinguishment of debt	130	(18)	(7)
Other non-operating income	57	3	18
Total other expense	(51)	(116)	(108)
Income from continuing operations before income taxes	(193)	313	565
Income tax benefit (expense)	111	(18)	475
Income (loss) from continuing operations	(82)	295	1,040
Income (loss) from discontinued operations, net of tax	1	(2)	88
Net income (loss)	(81)	293	1,128
Income attributable to noncontrolling interest	(41)	0	0
Net income (loss) attributable to Cliffs shareholders	$ (122)	$ 293	$ 1,128
Earnings (loss) per common share attributable to Cliffs shareholders - basic
Continuing operations	$ (0.32)	$ 1.07	$ 3.50
Discontinued operations	0	(0.01)	0.30
Decrease in basic earnings per common share	(0.32)	1.06	3.80
Earnings (loss) per common share attributable to Cliffs shareholders - diluted
Continuing operations	(0.32)	1.04	3.42
Discontinued operations	0	(0.01)	0.29
Decrease in diluted earnings per common share	$ (0.32)	$ 1.03	$ 3.71

Statements Of Condensed Conso_4

Statements Of Condensed Consolidated Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (81)	$ 293	$ 1,128
Other comprehensive income (loss):
Changes in pension and OPEB, net of tax	181	(35)	(17)
Changes in foreign currency translation	3	0	(225)
Changes in derivative financial instruments, net of tax	2	0	(3)
Total other comprehensive income (loss)	186	(35)	(245)
Comprehensive income	105	258	883
Comprehensive income attributable to noncontrolling interests	(41)	0	0
Comprehensive income attributable to Cliffs shareholders	$ 64	$ 258	$ 883

Statements Of Condensed Conso_5

Statements Of Condensed Consolidated Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES
Net income (loss)	$ (81)	$ 293	$ 1,128
Adjustments to reconcile net income (loss) to net cash provided (used) by operating activities:
Depreciation, depletion and amortization	308	85	89
Amortization of Inventory Step-up	96	0	0
Deferred income taxes	(101)	17	(461)
Loss (gain) on extinguishment of debt	(130)	18	7
Loss (gain) on derivatives	(104)	47	(110)
Gain on foreign currency translation	0	0	(228)
Other	11	66	21
Changes in operating assets and liabilities, net of business combination:
Receivables and other assets	(42)	255	52
Inventories	(146)	(136)	44
Pension and OPEB payments and contributions	(75)	(20)	(32)
Payables, accrued expenses and other liabilities	3	(62)	(31)
Net cash provided (used) by operating activities	(261)	563	479
INVESTING ACTIVITIES
Purchase of property, plant and equipment	(525)	(656)	(296)
Other investing activities	10	12	23
Net cash used by investing activities	(2,042)	(644)	(273)
FINANCING ACTIVITIES
Repurchase of common shares	0	(253)	(48)
Dividends paid	(41)	(72)	0
Proceeds from issuance of debt	1,763	721	0
Debt issuance costs	(76)	(7)	(2)
Repurchase of debt	(1,023)	(729)	(235)
Borrowings under credit facilities	2,060	0	0
Repayments under credit facilities	(550)	0	0
SunCoke Middletown distributions to noncontrolling interest owners	(61)	0	0
Other financing activities	(13)	(54)	(91)
Net cash provided (used) by financing activities	2,059	(394)	(376)
Decrease in cash and cash equivalents, including cash classified within other current assets related to discontinued operations	(3)	(5)	(17)
Less: decrease in cash and cash equivalents from discontinued operations, classified within other current assets	0	0	(2)
Net decrease in cash and cash equivalents	(244)	(475)	(172)
Cash and cash equivalents at beginning of year	353	823	978
Cash and cash equivalents at end of year	112	353	823
Net decrease in cash and cash equivalents	(241)	(470)	(155)
ArcelorMittal USA [Member]
INVESTING ACTIVITIES
Payments to Acquire Businesses, Net of Cash Acquired	(658)	0	0
AK Steel [Member]
INVESTING ACTIVITIES
Payments to Acquire Businesses, Net of Cash Acquired	$ (869)	$ 0	$ 0

Statements of Consolidated Chan

Statements of Consolidated Changes in Equity - USD ($) $ in Millions	Total	AK Steel [Member]	ArcelorMittal USA [Member]	Common Stock [Member]	Common Stock [Member]AK Steel [Member]	Common Stock [Member]ArcelorMittal USA [Member]	Capital in Excess of Par Value of Shares [Member]	Capital in Excess of Par Value of Shares [Member]AK Steel [Member]	Capital in Excess of Par Value of Shares [Member]ArcelorMittal USA [Member]	Retained Earnings [Member]	Common Shares in Treasury [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]AK Steel [Member]	Noncontrolling Interest [Member]ArcelorMittal USA [Member]
Balance, beginning of period (in shares) at Dec. 31, 2017				297,000,000
Balance, beginning of period at Dec. 31, 2017	$ (445)			$ 37			$ 3,934			$ (4,207)	$ (170)	$ (39)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of accounting standard | Accounting Standards Update 2014-09	34									34
Comprehensive income	883									1,128		(245)	0
Stock and other incentive plans (in shares)				1,000,000
Stock and other incentive plans, APIC							(17)
Stock and other incentive plans	14										31
Common stock repurchases (in shares)				(5,000,000)
Common stock repurchased (value)	$ (47)										(47)
Common Stock, Dividends, Per Share, Declared	$ 0.05
Common stock dividends	$ (15)									(15)
Balance, end of period (in shares) at Dec. 31, 2018				293,000,000
Balance, end of period at Dec. 31, 2018	424			$ 37			3,917			(3,060)	(186)	(284)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	258									293		(35)	0
Stock and other incentive plans (in shares)				2,000,000
Stock and other incentive plans, APIC							(44)
Stock and other incentive plans	4										48
Common stock repurchases (in shares)				(24,000,000)
Common stock repurchased (value)	$ (253)										(253)
Common Stock, Dividends, Per Share, Declared	$ 0.27
Common stock dividends	$ (75)									(75)
Balance, end of period (in shares) at Dec. 31, 2019	270,084,005			271,000,000
Balance, end of period at Dec. 31, 2019	$ 358			$ 37			3,873			(2,842)	(391)	(319)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	105									(122)		186	41
Stock and other incentive plans (in shares)				2,000,000
Stock and other incentive plans, APIC							(24)
Stock and other incentive plans	$ 13										37
Stock Issued During Period, Shares, Acquisitions					127,000,000	78,000,000
Stock Issued During Period, Value, Acquisitions		$ 948	$ 1,003		$ 16	$ 10		$ 602	$ 980
Noncontrolling Interest, Increase from Business Combination														$ 330	$ 13
Common Stock, Dividends, Per Share, Declared	$ 0.06
Common stock dividends	$ (25)									(25)
Distributions of partnership equity	$ (61)												(61)
Balance, end of period (in shares) at Dec. 31, 2020	477,517,372			478,000,000
Balance, end of period at Dec. 31, 2020	$ 2,341			$ 63			$ 5,431			$ (2,989)	$ (354)	$ (133)	$ 323

BASIS OF PRESENTATION AND SIGNI

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES	Business, Consolidation and Presentation Nature of Business Cliffs is the largest flat-rolled steel producer in North America. Founded in 1847 as a mine operator, we are also the largest producer of iron ore pellets in North America. In 2020, we acquired two major steelmakers, AK Steel and ArcelorMittal USA, vertically integrating our legacy iron ore business. Our fully-integrated portfolio includes custom-made pellets and HBI; flat-rolled carbon steel, stainless, electrical, plate, tinplate and long steel products; as well as carbon and stainless steel tubing, hot and cold stamping and tooling. Headquartered in Cleveland, Ohio, we employ approximately 25,000 people across our mining, steel and downstream manufacturing operations in the United States and Canada. Unless otherwise noted, discussion of our business and results of operations in this Annual Report on Form 10-K refers to our continuing operations. Acquisition of AK Steel On March 13, 2020, we consummated the AK Steel Merger, pursuant to which, upon the terms and subject to the conditions set forth in the AK Steel Merger Agreement, Merger Sub was merged with and into AK Steel, with AK Steel surviving the AK Steel Merger as a wholly owned subsidiary of Cleveland-Cliffs Inc. Refer to NOTE 3 - ACQUISITIONS for further information. AK Steel is a North American producer of flat-rolled carbon, stainless and electrical steel products, primarily for the automotive, infrastructure and manufacturing markets. These operations consist primarily of seven steelmaking and finishing plants, two cokemaking operations, three tube manufacturing plants and ten tooling and stamping operations. The acquisition of AK Steel transformed us into a vertically integrated producer of value-added iron ore and steel products. Acquisition of ArcelorMittal USA On December 9, 2020, pursuant to the terms of the AM USA Transaction Agreement, we purchased ArcelorMittal USA from ArcelorMittal. In connection with the closing of the AM USA Transaction, as contemplated by the terms of the AM USA Transaction Agreement, ArcelorMittal’s former joint venture partner in Kote and Tek exercised its put right pursuant to the terms of the Kote and Tek joint venture agreements. As a result, we purchased all of such joint venture partner’s interests in Kote and Tek. Following the closing of the AM USA Transaction, we own 100% of the interests in Kote and Tek. The assets of ArcelorMittal USA we acquired at the closing of the AM USA Transaction include six steelmaking facilities, eight finishing facilities, three cokemaking operations, two iron ore mining and pelletizing operations and one coal mining complex. Refer to NOTE 3 - ACQUISITIONS for further information. Business Operations We are vertically integrated from the mining of iron ore and coal; to production of metallics and coke; through iron making, steelmaking, rolling and finishing; and to downstream tubular components, stamping and tooling. We have the unique advantage as a steel producer of being fully or partially self-sufficient with our production of raw materials for steel manufacturing, which includes iron ore pellets, HBI and coking coal. We have updated our segment structure to coincide with our new business model and are organized into four operating segments based on differentiated products, Steelmaking, Tubular, Tooling and Stamping, and European Operations. Through the third quarter ended September 30, 2020, we had operated through two reportable segments – the Steel and Manufacturing segment and the Mining and Pelletizing segment. However, given the recent transformation of the business, beginning with our financial statements as of and for the year ended December 31, 2020, we primarily operate through one reportable segment – the Steelmaking segment. COVID-19 In response to the COVID-19 pandemic, we made various operational changes to adjust to the demand for our products. Although steel and iron ore production have been considered “essential” by the states in which we operate, certain of our facilities and construction activities were temporarily idled during the second quarter of 2020. Most of these temporarily idled facilities were restarted during the second quarter, and the remaining operations were restarted during the third quarter. Dearborn Works' hot strip mill, anneal and temper operations and AK Coal remain permanently idled as part of the permanent cost reduction efforts. Our Columbus and Monessen facilities acquired through the AM USA Transaction are temporarily idled due to the COVID-19 pandemic. Basis of Consolidation The condensed consolidated financial statements consolidate our accounts and the accounts of our wholly owned subsidiaries, all subsidiaries in which we have a controlling interest and VIEs for which we are the primary beneficiary. All intercompany transactions and balances are eliminated upon consolidation. Investments in Affiliates We have investments in several businesses accounted for using the equity method of accounting. These investments are included within our Steelmaking segment. We review an investment for impairment when circumstances indicate that a loss in value below its carrying amount is other than temporary. Investees and equity ownership percentages are presented below: Investee Equity Ownership Percentage Combined Metals of Chicago, LLC 40.0% Spartan Steel Coating, LLC 48.0% As of December 31, 2019, our 23% ownership in Hibbing was recorded as an equity method investment. As a result of the acquisition of ArcelorMittal USA, we acquired an additional 62.3% ownership interest in Hibbing. As of December 31, 2020, our ownership in the Hibbing joint venture was 85.3% and was fully consolidated within our operating results with a noncontrolling interest. We recorded a basis difference for Spartan Steel of $33 million as part of the AK Steel Merger. The basis difference relates to the excess of the fair value over the investee's carrying amount of property, plant and equipment and will be amortized over the remaining useful lives of the underlying assets. As of December 31, 2020, our investment in affiliates of $105 million was classified in Other non-current assets . As of December 31, 2019, our investment in affiliates of $18 million was classified in Other non-current liabilities. Significant Accounting Policies We consider the following policies to be beneficial in understanding the judgments involved in the preparation of our consolidated financial statements and the uncertainties that could impact our financial condition, results of operations and cash flows. Certain prior period amounts have been reclassified to conform with the current year presentation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Our mineral reserves; future realizable cash flow; environmental, reclamation and closure obligations; valuation of business combinations, long-lived assets, inventory, tax assets and post-employment, post-retirement and other employee benefit liabilities; reserves for contingencies and litigation require the use of various management estimates and assumptions. Actual results could differ from estimates. Management reviews its estimates on an ongoing basis. Changes in facts and circumstances may alter such estimates and affect the results of operations and financial position in future periods. Business Combinations Assets acquired and liabilities assumed in a business combination are recognized and measured based on their estimated fair values at the acquisition date, while the acquisition-related costs are expensed as incurred. Any excess of the purchase consideration when compared to the fair value of the net tangible and intangible assets acquired, if any, is recorded as goodwill. We engaged independent valuation specialists to assist with the determination of the fair value of assets acquired, liabilities assumed, noncontrolling interest, and goodwill, for the Acquisitions. If the initial accounting for the business combination is incomplete by the end of the reporting period in which the acquisition occurs, an estimate will be recorded. Subsequent to the acquisition date, and not later than one year from the acquisition date, we will record any material adjustments to the initial estimate based on new information obtained that would have existed as of the date of the acquisition. Any adjustment that arises from information obtained that did not exist as of the date of the acquisition will be recorded in the period the adjustment arises. Cash and Cash Equivalents Cash and cash equivalents include cash on hand and on deposit as well as all short-term securities held for the primary purpose of general liquidity. We consider investments in highly liquid debt instruments with an original maturity of three months or less from the date of acquisition and longer maturities when funds can be withdrawn in three months or less without a significant penalty to be cash equivalents. We routinely monitor and evaluate counterparty credit risk related to the financial institutions in which our short-term investment securities are held. Trade Accounts Receivable and Allowance for Credit Loss Trade accounts receivable are recorded at the point control transfers and represent the amount of consideration we expect to receive in exchange for transferred goods and do not bear interest. We establish provisions for expected lifetime losses on accounts receivable at the time a receivable is recorded based on historical experience, customer credit quality and forecasted economic conditions. We regularly review our accounts receivable balances and the allowance for credit loss and establish or adjust the allowance as necessary using the specific identification method in accordance with CECL. We evaluate the aggregation and risk characteristics of receivable pools and develop loss rates that reflect historical collections, current forecasts of future economic conditions over the time horizon we are exposed to credit risk, and payment terms or conditions that may materially affect future forecasts. Inventories Inventories are generally stated at the lower of cost or net realizable value using average cost, excluding depreciation and amortization. Certain iron ore inventories are stated at the lower of cost or market using the LIFO method. Refer to NOTE 2 - SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION for further information. Derivative Financial Instruments and Hedging Activities We are exposed to certain risks related to the ongoing operations of our business, including those caused by changes in commodity prices and energy rates. We have established policies and procedures, including the use of certain derivative instruments, to manage such risks, if deemed necessary. Derivative financial instruments are recognized as either assets or liabilities in the Statements of Consolidated Financial Position and measured at fair value. On the date a qualifying hedging instrument is executed, we designate the hedging instrument as a hedge of the variability of cash flows to be received or paid related to a forecasted transaction (cash flow hedge). We formally document all relationships between hedging instruments and hedged items, as well as our risk-management objective and strategy for undertaking various hedge transactions. This process includes linking all derivatives that are designated as cash flow hedges to specific firm commitments or forecasted transactions. We also formally assess, both at the hedge's inception and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows of the related hedged items. When it is determined that a derivative is not highly effective as a hedge, we discontinue hedge accounting prospectively and record all future changes in fair value in the period of the instrument's earnings or losses. For derivative instruments that have been designated as cash flow hedges, the changes in fair value are recorded in Accumulated other comprehensive loss . Amounts recorded in Accumulated other comprehensive loss are reclassified to earnings or losses in the period the underlying hedged transaction affects earnings or when the underlying hedged transaction is no longer reasonably possible of occurring. For derivative instruments that have not been designated as cash flow hedges, such as provisional pricing arrangements, changes in fair value are recorded in the period of the instrument's earnings or losses. Refer to Revenue Recognition below for discussion of derivatives recorded as a result of pricing terms in our sales contracts. Additionally, refer to NOTE 15 - DERIVATIVE INSTRUMENTS for further information. Property, Plant and Equipment Our properties are stated at the lower of cost less accumulated depreciation. Depreciation of plant and equipment is computed principally by the straight-line method based on estimated useful lives. Depreciation continues to be recognized when operations are idled temporarily. Depreciation and depletion are recorded over the following estimated useful lives: Asset Class Basis Life Land, land improvements and mineral rights Land and mineral rights Units of production Life of mine Land improvements Straight line 20 to 45 years Buildings Straight line 20 to 45 years Equipment Straight line/Double declining balance 3 to 20 years Refer to NOTE 6 - PROPERTY, PLANT AND EQUIPMENT for further information. Goodwill Goodwill represents the excess purchase price paid over the fair value of the net assets during an acquisition. Goodwill is not amortized, but is assessed for impairment on an annual basis on October 1 (or more frequently if necessary). Other Intangible Assets and Liabilities Intangible assets and liabilities are subject to periodic amortization on a straight-line basis over their estimated useful lives as follows: Type Basis Useful Life Intangible assets: Customer relationships Straight line 18 years Developed technology Straight line 17 years Trade names and trademarks Straight line 10 years Mining permits Straight line Life of mine Intangible liabilities: Above-market supply contracts Straight line Contract term Refer to NOTE 7 - GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES for further information. Leases We determine if an arrangement contains a lease at inception. We recognize right-of-use assets and lease liabilities associated with leases based on the present value of the future minimum lease payments over the lease term at the commencement date. Lease terms reflect options to extend or terminate the lease when it is reasonably certain that the option will be exercised. For short-term leases (leases with an initial lease term of 12 months or less), right-of-use assets and lease liabilities are not recognized in the consolidated balance sheet, and lease expense is recognized on a straight-line basis over the lease term. Refer to NOTE 13 - LEASE OBLIGATIONS for further information. Asset Impairment We monitor conditions that may affect the carrying value of our long-lived tangible and intangible assets when events and circumstances indicate that the carrying value of the asset groups may not be recoverable. In order to determine if assets have been impaired, assets are grouped and tested at the lowest level for which identifiable, independent cash flows are available ("asset group"). The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Fair value can be determined using a market approach, income approach or cost approach. For the years ended December 31, 2020, 2019 and 2018, no impairment indicators were present that would indicate the carrying value of any of our asset groups may not be recoverable; as a result, no impairment assessments were required. Fair Value Measurements ASC Topic 820, Fair Value Measurements and Disclosures , establishes a three-level valuation hierarchy for classification of fair value measurements. The valuation hierarchy is based upon the transparency of inputs to the valuation of an asset or liability as of the measurement date. Inputs refer broadly to the assumptions that market participants would use in pricing an asset or liability. Inputs may be observable or unobservable. Observable inputs are inputs that reflect the assumptions market participants would use in pricing the asset or liability developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our own views about the assumptions market participants would use in pricing the asset or liability developed based on the best information available in the circumstances. The three-tier hierarchy of inputs is summarized below: • Level 1 — Valuation is based upon quoted prices (unadjusted) for identical assets or liabilities in active markets. • Level 2 — Valuation is based upon quoted prices for similar assets and liabilities in active markets, or other inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument. • Level 3 — Valuation is based upon other unobservable inputs that are significant to the fair value measurement. The classification of assets and liabilities within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement in its entirety. Refer to NOTE 9 - FAIR VALUE OF FINANCIAL INSTRUMENTS and NOTE 10 - PENSIONS AND OTHER POSTRETIREMENT BENEFITS for further information. Pensions and Other Postretirement Benefits We offer defined benefit pension plans, defined contribution pension plans and OPEB plans, primarily consisting of retiree healthcare benefits as part of our total compensation and benefits programs. We recognize the funded or unfunded status of our pension and OPEB obligations on our December 31, 2020 and 2019 Statements of Consolidated Financial Position based on the difference between the market value of plan assets and the actuarial present value of our retirement obligations on that date, on a plan-by-plan basis. If the plan assets exceed the pension and OPEB obligations, the amount of the surplus is recorded as an asset; if the pension and OPEB obligations exceed the plan assets, the amount of the underfunded obligations is recorded as a liability. Year-end balance sheet adjustments to pension and OPEB assets and obligations are recorded as Accumulated other comprehensive loss in the Statements of Consolidated Financial Position. The actuarial estimates of the PBO and APBO incorporate various assumptions including the discount rates, the rates of increases in compensation, healthcare cost trend rates, mortality, retirement timing and employee turnover. The discount rate is determined based on the prevailing year-end rates for high-grade corporate bonds with a duration matching the expected cash flow timing of the benefit payments from the various plans. The remaining assumptions are based on our estimates of future events by incorporating historical trends and future expectations. The amount of net periodic cost that is recorded in the Statements of Consolidated Operations consists of several components including service cost, interest cost, expected return on plan assets, and amortization of previously unrecognized amounts. Service cost represents the value of the benefits earned in the current year by the participants. Interest cost represents the cost associated with the passage of time. Certain items, such as plan amendments, gains and/or losses resulting from differences between actual and assumed results for demographic and economic factors affecting the obligations and assets of the plans, and changes in other assumptions are subject to deferred recognition for income and expense purposes. The expected return on plan assets is determined utilizing the weighted average of expected returns for plan asset investments in various asset categories based on historical performance, adjusted for current trends. Service costs are classified within Cost of goods sold, Selling, general and administrative expenses and Miscellaneous – net while the interest cost, expected return on assets, amortization of prior service costs/credits, net actuarial gain/loss, and other costs are classified within Other non-operating income. Refer to NOTE 10 - PENSIONS AND OTHER POSTRETIREMENT BENEFITS for further information. Labor Agreements At December 31, 2020, we employed approximately 25,000 people, of which approximately 18,500 were represented by labor unions under various agreements. We have agreements that will expire at five locations in 2021 and sixteen locations in 2022. Workers at some of our North American facilities are covered by agreements with the USW or other unions that have various expiration dates. Asset Retirement Obligations Asset retirement obligations are recognized when incurred and recorded as liabilities at fair value. The fair value of the liability is determined as the discounted value of the expected future cash flows. The asset retirement obligation is accreted over time through periodic charges to earnings. In addition, the asset retirement cost is capitalized and amortized over the life of the related asset. Reclamation costs are adjusted periodically to reflect changes in the estimated present value resulting from the passage of time and revisions to the estimates of either the timing or amount of the reclamation costs. We review, on an annual basis, unless otherwise deemed necessary, the asset retirement obligation for each applicable operation in accordance with the provisions of ASC Topic 410, Asset Retirement and Environmental Obligations . We perform an in-depth evaluation of the liability every three years in addition to our routine annual assessments. Future reclamation costs for inactive operations are accrued based on management’s best estimate at the end of each period of the costs expected to be incurred at a site. Such cost estimates include, where applicable, ongoing maintenance and monitoring costs. Changes in estimates at inactive operations are reflected in earnings in the period an estimate is revised. Refer to NOTE 14 - ASSET RETIREMENT OBLIGATIONS for further information. Environmental Remediation Costs We have a formal policy for environmental protection and restoration. Certain of our operating activities are subject to various laws and regulations governing protection of the environment. We conduct our operations to protect the public health and environment and believe our operations are in compliance with applicable laws and regulations in all material respects. Our environmental liabilities, including obligations for known environmental remediation exposures, have been recognized based on the estimated cost of investigation and remediation at each site. If the cost can only be estimated as a range of possible amounts with no point in the range being more likely, the minimum of the range is accrued. Future expenditures are discounted unless the amount and timing of the cash disbursements cannot be reasonably estimated. It is possible that additional environmental obligations could be incurred, the extent of which cannot be assessed. Potential insurance recoveries have not been reflected in the determination of the liabilities. Refer to NOTE 21 - COMMITMENTS AND CONTINGENCIES for further information. Revenue Recognition Sales are recognized when our performance obligations are satisfied. Generally, our performance obligations are satisfied, control of our products is transferred and revenue is recognized at a single point in time, when title transfers to our customer for product shipped according to shipping terms. Shipping and other transportation costs charged to customers are treated as fulfillment activities and are recorded in both revenue and cost of sales at the time control is transferred to the customer. Refer to NOTE 4 - REVENUES for further information. Repairs and Maintenance Repairs, maintenance and replacement of components are expensed as incurred. The cost of major equipment overhauls is capitalized and depreciated over the estimated useful life, which is the period until the next scheduled overhauls. All other planned and unplanned repairs and maintenance costs are expensed when incurred. Share-Based Compensation The fair value of each performance share grant is estimated on the date of grant using a Monte Carlo simulation to forecast relative TSR performance. A correlation matrix of historical and projected stock prices was developed for both the Company and its predetermined peer group of mining and metals companies. The fair value assumes that the performance objective will be achieved. The expected term of the grant represents the time from the grant date to the end of the service period. We estimate the volatility of our common shares and that of the peer group of mining and metals companies using daily price intervals for all companies. The risk-free interest rate is the rate at the grant date on zero-coupon government bonds, with a term commensurate with the remaining performance period. The fair value of the restricted stock units is determined based on the closing price of our common shares on the grant date. Upon vesting of share-based compensation awards, we issue shares from treasury shares before issuing new shares. Forfeitures are recognized when they occur. The fair value of stock options is estimated on the date of grant using a Black-Scholes model using the grant date price of our common shares and option exercise price, and assumptions regarding the option’s expected term, the volatility of our common shares, the risk-free interest rate, and the dividend yield over the option’s expected term. Refer to NOTE 11 - STOCK COMPENSATION PLANS for additional information. Income Taxes Income taxes are based on income for financial reporting purposes, calculated using tax rates by jurisdiction, and reflect a current tax liability or asset for the estimated taxes payable or recoverable on the current year tax return and expected annual changes in deferred taxes. Any interest or penalties on income tax are recognized as a component of Income tax benefit (expense) . We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized within Net income (loss) in the period that includes the enactment date. We record net deferred tax assets to the extent we believe these assets will more likely than not be realized. In making such determination, we consider all available positive and negative evidence, including scheduled reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial results of operations. Accounting for uncertainty in income taxes recognized in the financial statements requires that a tax benefit from an uncertain tax position be recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on technical merits. See NOTE 12 - INCOME TAXES for further information. Discontinued Operations During 2018, we committed to a course of action leading to the permanent closure of the Asia Pacific Iron Ore mining operations and sold all of the assets of our Asia Pacific Iron Ore business through a series of sales to third parties. As a result of our exit, management determined that our Asia Pacific Iron Ore mining operations met the criteria to be classified as held for sale and a discontinued operation under ASC Topic 205, Presentation of Financial Statements . As such, all current and historical Asia Pacific Iron Ore operating results are classified within discontinued operations. Foreign Currency Our financial statements are prepared with the U.S. dollar as the reporting currency and the functional currency of all subsidiaries is the U.S. dollar, except for our European Operations for which the functional currency is the Euro. In August 2018, management determined that there were significant changes in economic factors related to our Australian subsidiaries. The change in economic factors was a result of the sale and conveyance of substantially all assets and liabilities of our Australian subsidiaries to third parties, representing a significant change in operations. As such, the functional currency for the Australian subsidiaries changed from the Australian dollar to the U.S. dollar and all remaining Australian denominated monetary balances will be remeasured prospectively through the Statements of Consolidated Operations. As a result of the liquidation of the Australian subsidiaries' assets, the historical impact of foreign currency translation recorded in Accumulated other comprehensive loss in the Statements of Consolidated Financial Position of $228 million was reclassified and recognized as a gain in Income (loss) from discontinued operations, net of tax in the Statements of Consolidated Operations for the year ended December 31, 2018. Earnings Per Share We present both basic and diluted EPS amounts for continuing operations and discontinued operations. Total basic EPS amounts are calculated by dividing Net income (loss) attributable to Cliffs shareholders , less the earnings allocated to our Series B Participating Redeemable Preferred Stock, by the weighted average number of common shares outstanding during the period presented. Total diluted EPS amounts are calculated by dividing Net income (loss) attributable to Cliffs shareholders by the weighted average number of common shares, common share equivalents under stock plans using the treasury-stock method, common share equivalents of the Series B Participating Redeemable Preferred Stock using the if-converted method and the calculated common share equivalents in excess of the conversion rate related to our 1.50% 2025 Convertible Senior Notes using the treasury-stock method. Common share equivalents are excluded from EPS computations in the periods in which they have an anti-dilutive effect. See NOTE 8 - DEBT AND CREDIT FACILITIES and NOTE 20 - EARNINGS PER SHARE for further information. Variable Interest Entities We assess whether we have a variable interest in legal entities in which we have a financial relationship and, if so, whether or not those entities are VIEs. A VIE is an entity with insufficient equity at risk for the entity to finance its activities without additional subordinated financial support or in which equity investors lack the characteristics of a controlling financial interest. If an entity is determined to be a VIE, we evaluate whether we are the primary beneficiary. The primary beneficiary analysis is a qualitative analysis based on power and economics. We conclude that we are the primary beneficiary and consolidate the VIE if we have both (i) the power to direct the activities of the VIE that most significantly influence the VIE's economic performance and (ii) the obligation to absorb losses of, or the right to receive benefits from, the VIE that could potentially be significant to the VIE. Refer to NOTE 19 - VARIABLE INTEREST ENTITIES for additional information. Recent Accounting Pronouncements Issued and Adopted On March 2, 2020, the SEC issued a final rule that amended the disclosure requirements related to certain registered securities under SEC Regulation S-X, Rule 3-10, which required separate financial statements for subsidiary issuers and guarantors of registered debt securities unless certain exceptions are met. The final rule replaces the previous requirement under Rule 3-10 to provide condensed consolidating financial information in the registrant’s financial statements with a requirement to provide alternative financial disclosures (which include summarized financi

SUPPLEMENTARY FINANCIAL STATEME

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION	12 Months Ended
	Dec. 31, 2020
Disclosure Text Block [Abstract]
Supplementary Financial Statement Information	Allowance for Credit Losses The following is a roll-forward of our allowance for credit losses associated with Accounts receivable, net : (In Millions) 2020 2019 Allowance for credit losses as of January 1 $ — $ — Increase in allowance (5) — Allowance for credit losses as of December 31 $ (5) $ — Inventories The following table presents the detail of our Inventories in the Statements of Consolidated Financial Position: (In Millions) Year Ended December 31, 2020 2019 Product inventories Finished and semi-finished goods $ 2,125 $ 114 Work-in-process — 69 Raw materials 1,431 9 Total product inventories 3,556 192 Manufacturing supplies and critical spares 272 125 Inventories $ 3,828 $ 317 The excess of current cost over LIFO cost of iron ore inventories was $104 million and $101 million at December 31, 2020 and 2019, respectively. As of December 31, 2020, the product inventory balance for iron ore inventories decreased, resulting in the liquidation of a LIFO layer. The effect of the inventory reduction was an increase in Cost of goods sold of $30 million in the Statements of Consolidated Operations for the year ended December 31, 2020. As of December 31, 2019, the product inventory balance for iron ore inventories increased, resulting in a LIFO increment in 2019. The effect of the inventory build was an increase in Inventories of $34 million in the Statements of Consolidated Financial Position for the year ended December 31, 2019. The allowance for obsolete and surplus items in supplies and other inventories was $13 million at both December 31, 2020 and 2019. Cash Flow Information A reconciliation of capital additions to cash paid for capital expenditures is as follows: (In Millions) Year Ended December 31, 2020 2019 2018 Capital additions $ 483 $ 690 $ 395 Less: Non-cash accruals (86) 15 94 Right-of-use assets - finance leases 44 29 8 Grants — (10) (3) Cash paid for capital expenditures including deposits $ 525 $ 656 $ 296 Cash payments (receipts) for interest and income taxes are as follows: (In Millions) 2020 2019 2018 Taxes paid on income $ 5 $ — $ 3 Income tax refunds (120) (118) (11) Interest paid on debt obligations net of capitalized interest 1 170 98 106 1 Capitalized interest was $53 million, $25 million and $7 million for the years ended December 31, 2020, 2019 and 2018, respectively. Non-Cash Investing and Financing Activities (In Millions) 2020 2019 2018 Fair value of common shares issued as part of consideration in connection with AM USA Transaction $ 990 $ — $ — Fair value of Series B Participating Redeemable Preferred Stock issued as part of consideration in connection with AM USA Transaction 738 — — Fair value of settlement of a pre-existing relationship as part of consideration in connection with AM USA Transaction 237 — — Fair value of common shares issued as consideration in connection with AK Steel Merger 618 — — Fair value of equity awards assumed in connection with AK Steel Merger 4 — — Discontinued Operations We had income from discontinued operations, net of tax of $88 million for the year ended December 31, 2018. During 2018, we sold all of the assets of our Asia Pacific Iron Ore mining operations, which had operating losses of $105 million for the year ended December 31, 2018. Additionally, as a result of the liquidation of the net assets of our Australian subsidiaries, the historical changes in foreign currency translation recorded in Accumulated other comprehensive loss totaling $228 million was reclassified and recognized as a gain in Income (loss) from discontinued operations, net of tax.

ACQUISITIONS

ACQUISITIONS	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
Business Combination Disclosure	In 2020, we acquired two major steelmakers, AK Steel and ArcelorMittal USA, vertically integrating our legacy iron ore business with steel production. Our fully-integrated portfolio includes custom-made pellets and HBI; flat-rolled carbon steel, stainless, electrical, plate, tinplate and long steel products; as well as carbon and stainless steel tubing, hot and cold stamping and tooling. The AK Steel Merger combined Cliffs, a producer of iron ore pellets, with AK Steel, a producer of flat-rolled carbon, stainless and electrical steel products, to create a vertically integrated producer of value-added iron ore and steel products. The AM USA Transaction transformed us into a fully-integrated steel enterprise with the size and scale to achieve improved through-the-cycle margins. We now have a presence across the entire steel manufacturing process, from mining to pelletizing to the development and production of finished high value steel products. The combination is expected to create significant opportunities to generate additional value from market trends across the entire steel value chain and enable more consistent, predictable performance through normal market cycles. Acquisition of ArcelorMittal USA Overview On December 9, 2020, pursuant to the terms of the AM USA Transaction Agreement, we purchased ArcelorMittal USA from ArcelorMittal. In connection with the closing of the AM USA Transaction, as contemplated by the terms of the AM USA Transaction Agreement, ArcelorMittal’s former joint venture partner in Kote and Tek exercised its put right pursuant to the terms of the Kote and Tek joint venture agreements. As a result, we purchased all of such joint venture partner’s interests in Kote and Tek. Following the closing of the AM USA Transaction, we own 100% of the interests in Kote and Tek. Following the acquisition, the operating results of ArcelorMittal USA are included in our consolidated financial statements. For the period subsequent to the acquisition (December 9, 2020 through December 31, 2020), ArcelorMittal USA generated Revenues of $446 million and a loss of $40 million included within Net income (loss) attributable to Cliffs shareholders , which included $21 million related to amortization of the fair value inventory step-up. Additionally, we incurred acquisition-related costs excluding severance costs of $26 million for the year ended December 31, 2020, which were recorded in Acquisition-related costs on the Statements of Consolidated Operations . The AM USA Transaction was accounted for under the acquisition method of accounting for business combinations. The fair value of the total purchase consideration was determined as follows: (In Millions) Fair value of Cliffs common shares issued $ 990 Fair value of Series B Participating Redeemable Preferred Stock issued 738 Fair value of settlement of a pre-existing relationship 237 Cash consideration (subject to customary working capital adjustments) 631 Total purchase consideration $ 2,596 The fair value of Cliffs common shares issued is calculated as follows: Number of Cliffs common shares issued 78,186,671 Closing price of Cliffs common share as of December 9, 2020 $ 12.66 Fair value of Cliffs common shares issued (in millions) $ 990 The fair value of Cliffs Series B Participating Redeemable Preferred Stock issued is calculated as follows: Number of Cliffs Series B Participating Redeemable Preferred Stock issued 583,273 Redemption price as of December 9, 2020 $ 1,266 Fair value of Cliffs Series B Participating Redeemable Preferred Stock issued (in millions) $ 738 The fair value of the estimated cash consideration is comprised of the following: (In Millions) Cash consideration pursuant to the AM USA Transaction Agreement $ 505 Cash consideration for purchase of the remaining JV partners' interest of Kote and Tek 182 Estimated total cash consideration receivable (56) Total estimated cash consideration $ 631 The cash portion of the purchase price is subject to customary working capital adjustments. The fair value of the settlement of a pre-existing relationship is comprised of the following: (In Millions) Accounts receivable $ 97 Freestanding derivative asset from customer supply agreement 140 Total fair value of settlement of a pre-existing relationship $ 237 Valuation Assumption and Preliminary Purchase Price Allocation We estimated fair values at December 9, 2020 for the preliminary allocation of consideration to the net tangible and intangible assets acquired and liabilities assumed in connection with the AM USA Transaction. During the measurement period, we will continue to obtain information to assist in finalizing the fair value of assets acquired and liabilities assumed, which may differ materially from these preliminary estimates. If we determine any measurement period adjustments are material, we will apply those adjustments, including any related impacts to net income, in the reporting period in which the adjustments are determined. We are in the process of conducting a valuation of the assets acquired and liabilities assumed related to the AM USA Transaction, most notably, inventories, personal and real property, mineral reserves, leases, investments, deferred taxes, asset retirement obligations, pension and OPEB liabilities, and the final allocation will be made when completed, including the result of any identified goodwill. Accordingly, the provisional measurements noted below are preliminary and subject to modification in the future. The preliminary purchase price allocation to assets acquired and liabilities assumed in the AM USA Transaction was: (In Millions) Cash and cash equivalents $ 35 Accounts receivable, net 349 Inventories 2,115 Income tax receivable, current 12 Other current assets 22 Property, plant and equipment 4,017 Other non-current assets 158 Accounts payable (758) Accrued employment costs (271) State and local taxes (76) Pension and OPEB liabilities, current (109) Other current liabilities (322) Pension and OPEB liabilities, non-current (3,195) Other non-current liabilities (598) Noncontrolling interest (13) Net identifiable assets acquired 1,366 Goodwill 1,230 Total net assets acquired $ 2,596 The goodwill resulting from the acquisition of ArcelorMittal USA primarily represents the growth opportunities in the automotive, construction, appliances, infrastructure and machinery and equipment markets, as well as any synergistic benefits to be realized from the AM USA Transaction and was assigned to our flat steel operations within our Steelmaking segment. Goodwill is expected to be deductible for U.S. federal income tax purposes. Acquisition of AK Steel Overview On March 13, 2020, pursuant to the AK Steel Merger Agreement, we completed the acquisition of AK Steel, in which we were the acquirer. As a result of the AK Steel Merger, each share of AK Steel common stock issued and outstanding immediately prior to the effective time of the AK Steel Merger (other than excluded shares) was converted into the right to receive 0.400 Cliffs common shares and, if applicable, cash in lieu of any fractional Cliffs common shares. Following the acquisition, the operating results of AK Steel are included in our consolidated financial statements. For the period subsequent to the acquisition (March 13, 2020 through December 31, 2020), AK Steel generated Revenues of $3,573 million and a loss of $302 million included within Net income (loss) attributable to Cliffs shareholders, which included $74 million and $35 million related to amortization of the fair value inventory step-up and severance costs, respectively. Additionally, we incurred acquisition-related costs excluding severance costs of $26 million for the year ended December 31, 2020, which were recorded in Acquisition-related costs on the Statements of Consolidated Operations . Refer to NOTE 8 - DEBT AND CREDIT FACILITIES for information regarding debt transactions executed in connection with the AK Steel Merger. The AK Steel Merger was accounted for under the acquisition method of accounting for business combinations. The acquisition date fair value of the consideration transferred totaled $1,535 million. The following tables summarize the consideration paid for AK Steel and the estimated fair values of the assets acquired and liabilities assumed at the acquisition date. The fair value of the total purchase consideration was determined as follows: (In Millions) Fair value of AK Steel debt $ 914 Fair value of Cliffs common shares issued for AK Steel outstanding common stock 618 Other 3 Total purchase consideration $ 1,535 The fair value of Cliffs common shares issued for outstanding shares of AK Steel common stock and with respect to Cliffs common shares underlying converted AK Steel equity awards that vested upon completion of the AK Steel Merger is calculated as follows: (In Millions, Except Per Share Amounts) Number of shares of AK Steel common stock issued and outstanding 317 Exchange ratio 0.400 Shares of Cliffs common shares issued to AK Steel stockholders 127 Price per share of Cliffs common shares $ 4.87 Fair value of Cliffs common shares issued for outstanding AK Steel common stock $ 618 The fair value of AK Steel's debt included in the consideration is calculated as follows: (In Millions) Credit Facility $ 590 7.50% Senior Secured Notes due July 2023 324 Fair value of debt included in consideration $ 914 Valuation Assumption and Preliminary Purchase Price Allocation We estimated fair values at March 13, 2020 for the preliminary allocation of consideration to the net tangible and intangible assets acquired and liabilities assumed in connection with the AK Steel Merger. During the measurement period, we will continue to obtain information to assist in finalizing the fair value of assets acquired and liabilities assumed, which may differ materially from these preliminary estimates. If we determine any measurement period adjustments are material, we will apply those adjustments, including any related impacts to net income, in the reporting period in which the adjustments are determined. We are in the process of conducting a valuation of the assets acquired and liabilities assumed related to the AK Steel Merger, most notably, personal and real property, leases, deferred taxes, asset retirement obligations and intangible assets and liabilities, and the final allocation will be made when completed, including the result of any identified goodwill. Accordingly, the provisional measurements noted below are preliminary and subject to modification in the future. The preliminary purchase price allocation to assets acquired and liabilities assumed in the AK Steel Merger was: (In Millions) Initial Allocation of Consideration Measurement Period Adjustments Updated Allocation Cash and cash equivalents $ 38 $ 1 $ 39 Accounts receivable, net 666 (2) 664 Inventories 1,563 (243) 1,320 Income tax receivable, current 3 — 3 Other current assets 65 (16) 49 Property, plant and equipment 2,184 128 2,312 Deferred income taxes — 30 30 Other non-current assets 475 (3) 472 Accounts payable (636) (8) (644) Accrued employment costs (94) 1 (93) State and local taxes (35) 4 (31) Pension and OPEB liabilities, current (75) (3) (78) Other current liabilities (201) 5 (196) Long-term debt (1,179) — (1,179) Pension and OPEB liabilities, non-current (873) 2 (871) Other non-current liabilities (507) 72 (435) Noncontrolling interest — (1) (1) Net identifiable assets acquired 1,394 (33) 1,361 Goodwill 141 33 174 Total net assets acquired $ 1,535 $ — $ 1,535 During the period subsequent to the AK Steel Merger, we made certain measurement period adjustments to the acquired assets and liabilities assumed due to clarification of information utilized to determine fair value during the measurement period. The Inventories measurement period adjustments of $243 million resulted in a favorable impact of $8 million to Cost of goods sold for the year ended December 31, 2020. The goodwill resulting from the acquisition of AK Steel was assigned to our downstream Tubular and Tooling and Stamping operating segments. Goodwill is calculated as the excess of the purchase price over the net identifiable assets recognized and primarily represents the growth opportunities in light weighting solutions to automotive customers, as well as any synergistic benefits to be realized. Goodwill from the AK Steel Merger is not expected be deductible for income tax purposes. The preliminary purchase price allocated to identifiable intangible assets and liabilities acquired was: (In Millions) Weighted Average Life (In Years) Intangible assets: Customer relationships $ 77 18 Developed technology 60 17 Trade names and trademarks 11 10 Total identifiable intangible assets $ 148 17 Intangible liabilities: Above-market supply contracts $ (71) 12 The above-market supply contracts relate to the long-term coke and energy supply agreements with SunCoke Energy, which includes SunCoke Middletown, a consolidated VIE. Refer to NOTE 19 - VARIABLE INTEREST ENTITIES for further information. Pro Forma Results The following table provides unaudited pro forma financial information, prepared in accordance with Topic 805, for the years ended December 31, 2020 and 2019, as if ArcelorMittal USA and AK Steel had been acquired as of January 1, 2019: (In Millions) Year Ended December 31, 2020 2019 Revenues $ 12,837 $ 17,163 Net income (loss) attributable to Cliffs shareholders (520) (11) The unaudited pro forma financial information has been calculated after applying our accounting policies and adjusting the historical results with pro forma adjustments, net of tax, that assume the Acquisitions occurred on January 1, 2019. Significant pro forma adjustments include the following: 1. The elimination of intercompany revenues between Cliffs and ArcelorMittal USA and AK Steel of $844 million and $1,499 million for the years ended December 31, 2020 and 2019, respectively. 2. The 2020 pro forma net loss was adjusted to exclude $96 million of non-recurring inventory acquisition accounting adjustments incurred during the year ended December 31, 2020. The 2019 pro forma net loss was adjusted to include $362 million of non-recurring inventory acquisition accounting adjustments for the year ended December 31, 2019. 3. The elimination of non-recurring transaction costs incurred by Cliffs, AK Steel and ArcelorMittal USA in connection with the Acquisitions were $93 million for the year ended December 31, 2020. The 2019 pro forma net loss was adjusted to include $93 million of non-recurring transaction cost adjustments for the year ended December 31, 2019. 4. The 2020 pro forma net loss was adjusted to exclude restructuring costs of $1,820 million of non-recurring costs incurred by ArcelorMittal USA prior to the AM USA Transaction. 5. The 2020 and 2019 pro forma net losses were adjusted to exclude $140 million and $129 million for the years ended December 31, 2020 and 2019, respectively, for the impact of reversal of the fees charged for management, financial and legal services under the Industrial Franchise Agreement with the former parent. 6. Total other pro forma adjustments included reduced expenses of $32 million for the year ended December 31, 2020, primarily due to decreased depreciation expense and pension and OPEB expense, offset partially by increased interest and amortization expense. 7. Total other pro forma adjustments included an expense of $76 million for the year ended December 31, 2019, primarily due to increased interest, amortization and pension and OPEB expense, offset partially by decreased depreciation expense. 8. The income tax impact of pro forma transaction adjustments that affect Net income (loss) attributable to Cliffs shareholders at a statutory rate of 24.3% resulted in an increased benefit to Income tax benefit (expense) of $170 million and $117 million for the years ended December 31, 2020 and 2019, respectively. The unaudited pro forma financial information does not reflect the potential realization of synergies or cost savings, nor does it reflect other costs relating to the integration of the acquired companies. This unaudited pro forma financial information should not be considered indicative of the results that would have actually occurred if the Acquisitions had been consummated on January 1, 2019, nor are they indicative of future results.

REVENUES

REVENUES	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer	We generate our revenue through product sales, in which shipping terms generally indicate when we have fulfilled our performance obligations and transferred control of products to our customer. Our revenue transactions consist of a single performance obligation to transfer promised goods. Our contracts with customers usually define the mechanism for determining the sales price, which is generally fixed upon transfer of control, but the contracts generally do not impose a specific quantity on either party. Quantities to be delivered to the customer are generally determined at a point near the date of delivery through purchase orders or other written instructions we receive from the customer. Spot market sales are made through purchase orders or other written instructions. We consider our performance obligation to be complete and recognize revenue when control transfers in accordance with shipping terms. Revenue is measured as the amount of consideration we expect to receive in exchange for transferring product. We reduce the amount of revenue recognized for estimated returns and other customer credits, such as discounts and volume rebates, based on the expected value to be realized. Payment terms are consistent with terms standard to the markets we serve. Sales taxes collected from customers are excluded from revenues. Prior to the AM USA Transaction, we had a supply agreement with ArcelorMittal USA, which included supplemental revenue or refunds based on the HRC price in the year the iron ore was consumed in ArcelorMittal USA's blast furnaces. As control transferred prior to consumption, the supplemental revenue was recorded in accordance with Topic 815. All sales occurring subsequent to the AM USA Transaction are intercompany and eliminated in consolidation. Included within Revenues related to Topic 815 for the supplemental revenue portion of the supply agreement is derivative revenue of $122 million, $78 million and $426 million for the years ended December 31, 2020, 2019 and 2018, respectively. The following table represents our Revenues by market: (In Millions) Year Ended December 31, 2020 2019 2018 Steelmaking: Automotive $ 2,062 $ — $ — Infrastructure and manufacturing 784 — — Distributors and converters 696 — — Steel producers 1 1,423 1,990 2,332 Total steelmaking 4,965 1,990 2,332 Other Businesses: Automotive 329 — — Infrastructure and manufacturing 34 — — Distributors and converters 26 — — Total Other Businesses 389 — — Total revenues $ 5,354 $ 1,990 $ 2,332 1 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020. The following table represents our consolidated Revenues by product line: (Dollars In Millions, Sales Volumes in Thousands) Year Ended December 31, 2020 2019 2018 Revenue Volume 1 Revenue Volume 1 Revenue Volume 1 Steelmaking: Hot-rolled steel $ 386 633 $ — — $ — — Cold-rolled steel 490 682 — — — — Coated steel 1,747 1,911 — — — — Stainless and electrical steel 868 416 — — — — Other steel products 92 141 — — — — Iron products 2 1,335 11,707 1,990 18,583 2,332 20,563 Other 47 N/A — — — — Total steelmaking 4,965 1,990 2,332 Other Businesses: Other 389 N/A — N/A — N/A Total revenues $ 5,354 $ 1,990 $ 2,332 1 Carbon steel products, stainless and electrical steel and plate steel volumes are stated in net tons. Iron product volumes are stated in long tons. 2 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020. Deferred Revenue The table below summarizes our deferred revenue balances: (In Millions) Deferred Revenue (Current) Deferred Revenue (Long-Term) 2020 2019 2020 2019 Opening balance as of January 1 $ 22 $ 21 $ 26 $ 39 Net increase (decrease) (15) 1 (26) (13) Closing balance as of December 31 $ 7 $ 22 $ — $ 26 Prior to the AK Steel Merger, our iron ore pellet sales agreement with Severstal Dearborn, LLC, subsequently assumed by AK Steel, required supplemental payments to be paid by the customer during the period from 2009 through 2013. Installment amounts received under this arrangement in excess of sales were classified as deferred revenue in the Statements of Consolidated Financial Position upon receipt of payment and the revenue was recognized over the term of the supply agreement, which had extended until 2022, in equal annual installments. As a result of the termination of that iron ore pellet sales agreement, we realized $35 million of deferred revenue, which was recognized within Realization of deferred revenue in the Statements of Consolidated Operations during the year ended December 31, 2020. We have certain other sales agreements that require customers to pay in advance. Payments received pursuant to these agreements prior to revenue being recognized are recorded as deferred revenue in Other current liabilities.

SEGMENT REPORTING

SEGMENT REPORTING	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
SEGMENT REPORTING	We are vertically integrated from the mining of iron ore and coal; to production of metallics and coke; through iron making, steelmaking, rolling, finishing; and to downstream tubing, stamping and tooling. We are organized into four operating segments based on our differentiated products - Steelmaking, Tubular, Tooling and Stamping, and European Operations. Our previous Mining and Pelletizing segment is included within the Steelmaking operating segment as iron ore pellets are a primary raw material for our steel products. We have one reportable segment - Steelmaking. The operating segment results of our Tubular, Tooling and Stamping, and European Operations that do not constitute reportable segments are combined and disclosed in the Other Businesses category. Our Steelmaking segment is the largest flat-rolled steel producer supported by being the largest iron ore pellet producer in North America, primarily serving the automotive, infrastructure and manufacturing, and distributors and converters markets. Our Other Businesses primarily include the operating segments that provide customer solutions with carbon and stainless steel tubing products, advanced-engineered solutions, tool design and build, hot- and cold-stamped steel components, and complex assemblies. All intersegment transactions were eliminated in consolidation. We evaluate performance on an operating segment basis, as well as a consolidated basis, based on Adjusted EBITDA, which is a non-GAAP measure. This measure is used by management, investors, lenders and other external users of our financial statements to assess our operating performance and to compare operating performance to other companies in the steel industry. In addition, management believes Adjusted EBITDA is a useful measure to assess the earnings power of the business without the impact of capital structure and can be used to assess our ability to service debt and fund future capital expenditures in the business. Our results by segment are as follows: (In Millions) Year Ended December 31, 2020 2019 2018 Revenues: Steelmaking 1 $ 4,965 $ 1,990 $ 2,332 Other Businesses 389 — — Total revenues $ 5,354 $ 1,990 $ 2,332 Adjusted EBITDA: Steelmaking $ 433 $ 636 $ 872 Other Businesses 47 — — Corporate and eliminations (127) (111) (106) Total Adjusted EBITDA $ 353 $ 525 $ 766 1 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020. The following table provides a reconciliation of our consolidated Net income (loss) to total Adjusted EBITDA: (In Millions) Year Ended December 31, 2020 2019 2018 Net income (loss) $ (81) $ 293 $ 1,128 Less: Interest expense, net (238) (101) (121) Income tax benefit (expense) 111 (18) 460 Depreciation, depletion and amortization (308) (85) (89) 354 497 878 Less: EBITDA from noncontrolling interests 1 56 — — Gain (loss) on extinguishment of debt 130 (18) (7) Severance costs (38) (2) — Acquisition-related costs excluding severance costs (52) (7) — Amortization of inventory step-up (96) — — Impact of discontinued operations 1 (1) 121 Foreign exchange remeasurement — — (1) Impairment of other long-lived assets — — (1) Total Adjusted EBITDA $ 353 $ 525 $ 766 1 EBITDA of noncontrolling interests includes $41 million for income and $15 million for depreciation, depletion and amortization for the year ended December 31, 2020. The following table summarizes our depreciation, depletion and amortization and capital additions by segment: (In Millions) Year Ended December 31, 2020 2019 2018 Depreciation, depletion and amortization: Steelmaking $ 276 $ 80 $ 68 Other Businesses 27 — — Corporate 5 5 6 Total depreciation, depletion and amortization $ 308 $ 85 $ 74 Capital additions 1: Steelmaking $ 436 $ 687 $ 393 Other Businesses 45 — — Corporate 2 3 2 Total capital additions $ 483 $ 690 $ 395 1 Refer to NOTE 2 - SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION for additional information. The following summarizes our assets by segment: (In Millions) December 31, 2020 2019 Assets: Steelmaking $ 15,849 $ 2,557 Other Businesses 239 — Total segment assets 16,088 2,557 Corporate 683 947 Total assets $ 16,771 $ 3,504 Included in the consolidated financial statements are the following amounts relating to geographic location based on product destination: (In Millions) 2020 2019 2018 Revenues: United States $ 4,580 $ 1,505 $ 1,847 Canada 602 448 395 Other countries 172 37 90 Total revenues $ 5,354 $ 1,990 $ 2,332 Property, plant and equipment, net: United States $ 8,647 $ 1,929 $ 1,286 Canada 91 — — Other countries 5 — — Total property, plant and equipment, net $ 8,743 $ 1,929 $ 1,286

PROPERTY, PLANT AND EQUIPMENT

PROPERTY, PLANT AND EQUIPMENT	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
PROPERTY, PLANT AND EQUIPMENT	The following table indicates the carrying value of each of the major classes of our depreciable assets: (In Millions) December 31, 2020 2019 Land, land improvements, and mineral rights $ 1,213 $ 582 Buildings 703 158 Equipment 6,786 1,456 Other 151 101 Construction in progress 1,364 730 Total property, plant and equipment 1 10,217 3,027 Allowance for depreciation and depletion (1,474) (1,098) Property, plant, and equipment, net $ 8,743 $ 1,929 1 Includes right-of-use assets related to finance leases of $361 million and $49 million as of December 31, 2020 and 2019, respectively. We recorded depreciation expense of $298 million, $77 million and $66 million for the years ended December 31, 2020, 2019 and 2018, respectively. We recorded capitalized interest into property, plant and equipment of $53 million, $25 million and $7 million during the years ended December 31, 2020, 2019 and 2018, respectively. The net book value of the mineral and land rights are as follows: (In Millions) December 31, 2020 2019 Mineral rights: Cost $ 773 $ 537 Depletion (142) (134) Net mineral rights $ 631 $ 403 Land rights $ 361 $ 12 We recorded depletion expense of $8 million, $8 million and $7 million for the years ended December 31, 2020, 2019, and 2018, respectively.

GOODWILL AND INTANGIBLE ASSETS

GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets and Liabilities Disclosure	Goodwill The following is a summary of goodwill by segment: (In Millions) December 31, 2020 2019 Steelmaking $ 1,232 $ 2 Other Businesses 174 — Total goodwill $ 1,406 $ 2 The increase in the balance of goodwill in our Steelmaking segment as of December 31, 2020, compared to December 31, 2019, is due to the preliminary assignment of $1,230 million to Goodwill in 2020 based on the preliminary purchase price allocation for the acquisition of ArcelorMittal USA. The increase in the balance of goodwill for our Other Businesses as of December 31, 2020, compared to December 31, 2019, is due to the preliminary assignment of $174 million to Goodwill in 2020 based on the preliminary purchase price allocation for the acquisition of AK Steel, which was attributable to our Tubular and Tooling and Stamping operating segments. Intangible Assets and Liabilities The following is a summary of our intangible assets and liabilities: (In Millions) Classification 1 Gross Amount Accumulated Amortization Net Amount As of December 31, 2020 Intangible assets: Customer relationships Other non-current assets $ 77 $ (3) $ 74 Technology Other non-current assets 60 (3) 57 Trade names and trademarks Other non-current assets 11 (1) 10 Mining permits Other non-current assets 72 (25) 47 Total intangible assets $ 220 $ (32) $ 188 Intangible liabilities: Above-market supply contracts Other non-current liabilities $ (71) $ 7 $ (64) As of December 31, 2019 Intangible assets: Mining permits Other non-current assets $ 72 $ (24) $ 48 1 Amortization of intangible liabilities related to above-market supply contracts and intangible assets related to mining permits is recognized in Cost of goods sold . Amortization of all other intangible assets is recognized in Selling, general and administrative expenses. Amortization expense related to intangible assets was $8 million and $1 million for the years ended December 31, 2020 and 2019, respectively. Estimated future amortization expense related to intangible assets is $10 million annually for the years 2021 through 2025.

DEBT AND CREDIT FACILITIES

DEBT AND CREDIT FACILITIES	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
DEBT AND CREDIT FACILITIES	The following represents a summary of our long-term debt: (In Millions) December 31, 2020 Debt Instrument Issuer 1 Annual Effective Interest Rate Total Principal Amount Unamortized Debt Issuance Costs Unamortized Premiums (Discounts) Total Debt Senior Secured Notes: 4.875% 2024 Senior Secured Notes Cliffs 5.00% $ 395 $ (3) $ (1) $ 391 9.875% 2025 Senior Secured Notes Cliffs 10.57% 955 (8) (25) 922 6.75% 2026 Senior Secured Notes Cliffs 6.99% 845 (20) (9) 816 Senior Unsecured Notes: 7.625% 2021 AK Senior Notes AK Steel 7.33% 34 — — 34 7.50% 2023 AK Senior Notes AK Steel 6.17% 13 — — 13 6.375% 2025 Senior Notes Cliffs 8.11% 64 — (4) 60 6.375% 2025 AK Senior Notes AK Steel 8.11% 29 — (2) 27 1.50% 2025 Convertible Senior Notes Cliffs 6.26% 296 (4) (49) 243 5.75% 2025 Senior Notes Cliffs 6.01% 396 (3) (4) 389 7.00% 2027 Senior Notes Cliffs 9.24% 73 — (8) 65 7.00% 2027 AK Senior Notes AK Steel 9.24% 56 — (6) 50 5.875% 2027 Senior Notes Cliffs 6.49% 556 (4) (18) 534 6.25% 2040 Senior Notes Cliffs 6.34% 263 (2) (3) 258 IRBs due 2024 to 2028 AK Steel Various 92 — 2 94 EDC Revolving Facility 3 * 3.25% 40 — — 18 ABL Facility 3 Cliffs 2 2.15% 3,500 — — 1,510 Total debt 5,424 Less: current debt 34 Total long-term debt $ 5,390 * Our subsidiaries, Fleetwood Metal Industries Inc. and The Electromac Group Inc., are the borrowers under the EDC Revolving Facility. 1 Unless otherwise noted, references in this column and throughout this Note 8 - DEBT AND CREDIT FACILITIES to "Cliffs" are to Cleveland-Cliffs Inc., and references to "AK Steel" are to AK Steel Corporation (n/k/a Cleveland-Cliffs Steel Corporation). 2 Refers to Cleveland-Cliffs Inc. as borrower under our ABL Facility. 3 The total principal amounts for the indicated credit facilities are stated at their respective maximum borrowing capacities. (In Millions) December 31, 2019 Debt Instrument Issuer 1 Annual Effective Interest Rate Total Principal Amount Debt Issuance Costs Unamortized Discounts Total Debt Senior Secured Notes: 4.875% 2024 Senior Notes Cliffs 5.00% $ 400 $ (5) $ (2) $ 393 Senior Unsecured Notes: 1.50% 2025 Convertible Senior Notes Cliffs 6.26% 316 (4) (65) 247 5.75% 2025 Senior Notes Cliffs 6.01% 473 (3) (6) 464 5.875% 2027 Senior Notes Cliffs 6.49% 750 (6) (27) 717 6.25% 2040 Senior Notes Cliffs 6.34% 298 (2) (3) 293 Former ABL Facility Cliffs 2 N/A 450 N/A N/A — Total long-term debt $ 2,114 1 Unless otherwise noted, references in this column to "Cliffs" are to Cleveland-Cliffs Inc. 2 Refers to Cleveland-Cliffs Inc. and certain of its subsidiaries as borrowers under our Former ABL Facility. Outstanding Senior Secured Notes 9.875% 2025 Senior Secured Notes On April 17, 2020, we entered into an indenture among Cliffs, the guarantors party thereto and U.S. Bank National Association, as trustee and notes collateral agent, relating to the issuance by Cliffs of $400 million aggregate principal amount of 9.875% 2025 Senior Secured Notes issued at 94.5% of face value. On April 24, 2020, we issued an additional $555 million aggregate principal amount of 9.875% 2025 Senior Secured Notes issued at 99.0% of face value. These additional notes are of the same class and series as, and otherwise identical to, the 9.875% 2025 Senior Secured Notes issued on April 17, 2020, other than with respect to the date of issuance and issue price. The 9.875% 2025 Senior Secured Notes were issued in private placement transactions exempt from the registration requirements of the Securities Act. The 9.875% 2025 Senior Secured Notes bear interest at a rate of 9.875% per annum, payable semi-annually in arrears on April 17 and October 17 of each year, commencing on October 17, 2020. The 9.875% 2025 Senior Secured Notes will mature on October 17, 2025 and are secured senior obligations of Cliffs. The 9.875% 2025 Senior Secured Notes are jointly and severally and fully and unconditionally guaranteed on a senior secured basis by substantially all of our material domestic subsidiaries and are secured (subject in each case to certain exceptions and permitted liens) by (i) a first-priority lien, on a pari passu basis with the 6.75% 2026 Senior Secured Notes and 4.875% 2024 Senior Secured Notes, on substantially all of our assets and the assets of the guarantors, other than the ABL Collateral (as defined below), and (ii) a second-priority lien on the ABL Collateral, which is junior to a first-priority lien for the benefit of the lenders under our ABL Facility. The 9.875% 2025 Senior Secured Notes may be redeemed, in whole or in part, at any time at our option upon not less than 30, and not more than 60, days' prior notice sent to the holders of the 9.875% 2025 Senior Secured Notes. The following is a summary of redemption prices for our 9.875% 2025 Senior Secured Notes: Redemption Period Redemption Price 1 Restricted Amount Prior to October 17, 2022 - using proceeds of equity issuance 109.875 % Up to 35% of original aggregate principal Prior to October 17, 2022 2 100.000 Beginning on October 17, 2022 107.406 Beginning on April 17, 2023 104.938 Beginning on April 17, 2024 102.469 Beginning on April 17, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to, but excluding, the redemption date. 2 Plus a "make-whole" premium. In addition, if a change in control triggering event, as defined in the indenture, occurs with respect to the 9.875% 2025 Senior Secured Notes, we will be required to offer to purchase the notes at a purchase price equal to 101% of their principal amount, plus accrued and unpaid interest, if any, to, but not including, the date of purchase. The terms of the 9.875% 2025 Senior Secured Notes contain certain customary covenants; however, there are no financial covenants. 6.75% 2026 Senior Secured Notes On March 13, 2020, we entered into an indenture among Cliffs, the guarantors party thereto and U.S. Bank National Association, as trustee and notes collateral agent, relating to the issuance of $725 million aggregate principal amount of 6.75% 2026 Senior Secured Notes issued at 98.783% of face value. On June 19, 2020, we issued an additional $120 million aggregate principal amount of 6.75% 2026 Senior Secured Notes issued at 99.25% of face value. These additional notes are of the same class and series as, and otherwise identical to, the 6.75% 2026 Senior Secured Notes issued on March 13, 2020, other than with respect to the date of issuance and issue price. The 6.75% 2026 Senior Secured Notes were issued in private placement transactions exempt from the registration requirements of the Securities Act. The 6.75% 2026 Senior Secured Notes bear interest at a rate of 6.75% per annum, payable semi-annually in arrears on March 15 and September 15 of each year, commencing on September 15, 2020. The 6.75% 2026 Senior Secured Notes mature on March 15, 2026 and are secured senior obligations of Cliffs. The 6.75% 2026 Senior Secured Notes are jointly and severally and fully and unconditionally guaranteed on a senior secured basis by substantially all of our material domestic subsidiaries and are secured (subject in each case to certain exceptions and permitted liens) by (i) a first-priority lien, on a pari passu basis with the 4.875% 2024 Senior Secured Notes and 9.875% 2025 Senior Secured Notes, on substantially all of our assets and the assets of the guarantors, other than the ABL Collateral, and (ii) a second-priority lien on the ABL Collateral, which is junior to a first-priority lien for the benefit of the lenders under our ABL Facility. The 6.75% 2026 Senior Secured Notes may be redeemed, in whole or in part, at any time at our option upon not less than 30, and not more than 60, days' prior notice sent to the holders of the 6.75% 2026 Senior Secured Notes. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for our 6.75% 2026 Senior Secured Notes: Redemption Period Redemption Price 1 Restricted Amount Prior to March 15, 2022 - using proceeds of equity issuance 106.750 % Up to 35% of original aggregate principal Prior to March 15, 2022 2 100.000 Beginning on March 15, 2022 105.063 Beginning on March 15, 2023 103.375 Beginning on March 15, 2024 101.688 Beginning on March 15, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to, but excluding, the redemption date. 2 Plus a "make-whole" premium. In addition, if a change in control triggering event, as defined in the indenture, occurs with respect to the 6.75% 2026 Senior Secured Notes, we will be required to offer to purchase the notes at a purchase price equal to 101% of their principal amount, plus accrued and unpaid interest, if any, to, but not including, the date of purchase. The terms of the 6.75% 2026 Senior Secured Notes contain certain customary covenants; however, there are no financial covenants. 4.875% 2024 Senior Secured Notes Our 4.875% 2024 Senior Secured Notes bear interest at a rate of 4.875% per annum, which is payable semi-annually in arrears on January 15 and July 15 of each year. The 4.875% 2024 Senior Secured Notes mature on January 15, 2024 and are secured senior obligations of Cliffs. Our 4.875% 2024 Senior Secured Notes are jointly and severally and fully and unconditionally guaranteed on a senior secured basis by substantially all of our material domestic subsidiaries and are secured (subject in each case to certain exceptions and permitted liens) by (i) a first-priority lien, on a pari passu basis with the 9.875% 2025 Senior Secured Notes and 6.75% 2026 Senior Secured Notes, on substantially all of our assets and the assets of the guarantors, other than the ABL Collateral, and (ii) a second-priority lien on the ABL Collateral, which is junior to a first-priority lien for the benefit of the lenders under our ABL Facility. The 4.875% 2024 Senior Secured Notes may be redeemed, in whole or in part, at any time at our option upon not less than 30, and not more than 60, days' prior notice sent to the holders of the 4.875% 2024 Senior Secured Notes. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for our 4.875% 2024 Senior Secured Notes: Redemption Period Redemption Price 1 Beginning on January 15, 2021 102.438 % Beginning on January 15, 2022 101.219 Beginning on January 15, 2023 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. In addition, if a change in control triggering event, as defined in the indenture, occurs with respect to the 4.875% 2024 Senior Secured Notes, we will be required to offer to purchase the notes at a purchase price equal to 101% of their principal amount, plus accrued and unpaid interest, if any, to, but not including, the date of purchase. The terms of the 4.875% 2024 Senior Secured Notes contain certain covenants; however, there are no financial covenants. Outstanding Senior Unsecured Notes Cliffs Senior Notes exchanged for AK Steel Corporation Senior Notes On March 16, 2020, we entered into indentures, in each case among Cliffs, the guarantors party thereto and U.S. Bank National Association, as trustee, relating to the issuance by Cliffs of $232 million aggregate principal amount of 6.375% 2025 Senior Notes and $335 million aggregate principal amount of 7.00% 2027 Senior Notes. The new notes were issued in exchange for equal aggregate principal amounts of 6.375% 2025 AK Senior Notes and 7.00% 2027 AK Senior Notes, respectively, issued by AK Steel Corporation. The 6.375% 2025 Senior Notes and 7.00% 2027 Senior Notes were issued in connection with the AK Steel Merger pursuant to private exchange offers exempt from the registration requirements of the Securities Act made by Cliffs. Pursuant to the registration rights agreements executed in connection with the issuance of the new notes, we agreed to file registration statements with the SEC with respect to registered offers to exchange the 6.375% 2025 Senior Notes and 7.00% 2027 Senior Notes for publicly registered notes within 365 days of the closing date, with all significant terms and conditions remaining the same. The 6.375% 2025 Senior Notes and 7.00% 2027 Senior Notes are unsecured obligations and rank equally in right of payment with all of our existing and future unsecured and unsubordinated indebtedness. The notes are guaranteed on a senior unsecured basis by our material direct and indirect wholly owned domestic subsidiaries and, therefore, are structurally senior to any of our existing and future indebtedness that is not guaranteed by such guarantors and are structurally subordinated to all existing and future indebtedness and other liabilities of our subsidiaries that do not guarantee the notes. In addition, if a change in control triggering event, as defined in the indentures, occurs with respect to the 6.375% 2025 Senior Notes or 7.00% 2027 Senior Notes, we will be required to offer to purchase the notes at a purchase price equal to 101% of their principal amount, plus accrued and unpaid interest, if any, to, but not including, the date of purchase. The terms of the 6.375% 2025 Senior Notes and 7.00% 2027 Senior Notes contain certain customary covenants; however, there are no financial covenants. 6.375% 2025 Senior Notes The 6.375% 2025 Senior Notes bear interest at a rate of 6.375% per annum, payable semi-annually in arrears on April 15 and October 15 of each year, commencing on April 15, 2020. The 6.375% 2025 Senior Notes mature on October 15, 2025. The 6.375% 2025 Senior Notes may be redeemed, in whole or in part, at any time at our option upon not less than 30, and not more than 60, days' prior notice sent to the holders of the 6.375% 2025 Senior Notes. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for our 6.375% 2025 Senior Notes: Redemption Period Redemption Price 1 Beginning on October 15, 2020 103.188 % Beginning on October 15, 2021 101.594 Beginning on October 15, 2022 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. 7.00% 2027 Senior Notes The 7.00% 2027 Senior Notes bear interest at a rate of 7.00% per annum, payable semi-annually in arrears on March 15 and September 15 of each year, commencing on September 15, 2020. The 7.00% 2027 Senior Notes mature on March 15, 2027. The 7.00% 2027 Senior Notes may be redeemed, in whole or in part, at any time at our option upon not less than 30, and not more than 60, days' prior notice sent to the holders of the 7.00% 2027 Senior Notes. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for our 7.00% 2027 Senior Notes: Redemption Period Redemption Price 1 Prior to March 15, 2022 2 100.000 % Beginning on March 15, 2022 103.500 Beginning on March 15, 2023 102.333 Beginning on March 15, 2024 101.167 Beginning on March 15, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. 2 Plus a "make-whole" premium. AK Steel Corporation Unsecured Senior Notes As of December 31, 2020, AK Steel Corporation had outstanding a total of $132 million aggregate principal amount of 7.625% 2021 AK Senior Notes, 7.50% 2023 AK Senior Notes, 6.375% 2025 AK Senior Notes and 7.00% 2027 AK Senior Notes. These senior notes are unsecured obligations and rank equally in right of payment with AK Steel Corporation's guarantees of Cliffs' unsecured and unsubordinated indebtedness. These notes contain no financial covenants. The 7.625% 2021 AK Senior Notes, 7.50% 2023 AK Senior Notes, 6.375% 2025 AK Senior Notes and 7.00% 2027 AK Senior Notes may be redeemed, in whole or in part, at any time at our option upon not less than 30, and not more than 60, days' prior notice sent to the respective holders of such notes. We may redeem the 7.625% 2021 AK Senior Notes at 100.000% of their principal amount, together with all accrued and unpaid interest to the date of redemption. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for the 7.50% 2023 AK Senior Notes: Redemption Period Redemption Price 1 Beginning on July 15, 2020 101.875 % Beginning on July 15, 2021 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for the 6.375% 2025 AK Senior Notes: Redemption Period Redemption Price 1 Beginning on October 15, 2020 103.188 % Beginning on October 15, 2021 101.594 Beginning on October 15, 2022 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for the 7.00% 2027 AK Senior Notes: Redemption Period Redemption Price 1 Prior to March 15, 2022 2 100.000 % Beginning on March 15, 2022 103.500 Beginning on March 15, 2023 102.333 Beginning on March 15, 2024 101.167 Beginning on March 15, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. 2 Plus a "make-whole" premium. 1.50% 2025 Convertible Senior Notes The 1.50% 2025 Convertible Senior Notes bear interest at a rate of 1.50% per year, payable semiannually in arrears on January 15 and July 15 of each year. The 1.50% 2025 Convertible Senior Notes mature on January 15, 2025. The 1.50% 2025 Convertible Senior Notes are senior unsecured obligations and rank senior in right of payment to any of our indebtedness that is expressly subordinated in right of payment to the 1.50% 2025 Convertible Senior Notes; equal in right of payment to any of our unsecured indebtedness that is not so subordinated; effectively junior in right of payment to any of our secured indebtedness to the extent of the value of the assets securing such indebtedness; and structurally junior to all indebtedness and other liabilities (including trade payables) of our subsidiaries. The terms of the 1.50% 2025 Convertible Senior Notes contain certain customary covenants; however, there are no financial covenants. Holders may convert their 1.50% 2025 Convertible Senior Notes at their option at any time prior to the close of business on the business day immediately preceding July 15, 2024, only under the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on March 31, 2018, if the last reported sale price of our common shares, par value $0.125 per share, for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five-business day period after any five-consecutive trading day period (the “measurement period”) in which the trading price per $1,000 principal amount of 1.50% 2025 Convertible Senior Notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of our common shares and the conversion rate on each such trading day; (3) if we call the notes for redemption, at any time prior to the close of business on the scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On or after July 15, 2024 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 1.50% 2025 Convertible Senior Notes at any time, regardless of the foregoing circumstances. Upon conversion, we will pay or deliver, as the case may be, cash, common shares or a combination of cash and common shares, at our election. Upon the issuance of the 1.50% 2025 Convertible Senior Notes the initial conversion rate was 122.4365 common shares per $1,000 principal, with a conversion price of $8.17 per common share. The conversion rate is subject to adjustment in some circumstances, including the payment of dividends on common shares, but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the maturity date, or if we deliver a notice of redemption, we will, in certain circumstances, increase the conversion rate for a holder who elects to convert its 1.50% 2025 Convertible Senior Notes in connection with such a corporate event or notice of redemption, as the case may be. As of December 31, 2020, the conversion rate was 129.2985 common shares per $1,000 principal amount of 1.50% 2025 Convertible Senior Notes. We may not redeem the 1.50% 2025 Convertible Senior Notes prior to January 15, 2022. We may redeem all or any portion of the 1.50% 2025 Convertible Senior Notes, for cash at our option on or after January 15, 2022 if the last reported sale price of our common shares has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive) during any 30-consecutive trading day period (including the last trading day of such period) ending on, and including, the trading day immediately preceding the date on which we provide notice of redemption at a redemption price equal to 100% of the principal amount of the 1.50% 2025 Convertible Senior Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. It is our current intent to settle conversions through combination settlement or fully in cash. Our ability to settle conversions through combination settlement and cash settlement will be subject to restrictions in the agreement governing our ABL Facility and may be subject to restrictions in agreements governing our future debt. If we undergo a fundamental change as defined in the indenture, holders may require us to repurchase for cash all or any portion of their 1.50% 2025 Convertible Senior Notes at a fundamental change repurchase price equal to 100% of the principal amount of the 1.50% 2025 Convertible Senior Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date. In accounting for the issuance of the notes, we separated the 1.50% 2025 Convertible Senior Notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of similar liabilities that did not have associated convertible features. The carrying amount of the equity component of $86 million representing the conversion option was determined by deducting the fair value of the liability component from the par value of the notes. The difference represents the debt discount that is amortized to interest expense over the term of the notes. The equity component is not remeasured as long as it continues to qualify for equity classification. Other Outstanding Unsecured Senior Notes The following represents a summary of our other unsecured senior notes' maturity and interest payable due dates: Debt Instrument Maturity Interest Payable 5.75% 2025 Senior Notes March 1, 2025 March 1 and September 1 5.875% 2027 Senior Notes June 1, 2027 June 1 and December 1 6.25% 2040 Senior Notes October 1, 2040 April 1 and October 1 The senior notes are unsecured obligations and rank equally in right of payment with all our other existing and future unsecured and unsubordinated indebtedness. There are no subsidiary guarantees of the interest and principal amounts for the 6.25% 2040 Senior Notes. The 5.75% 2025 Senior Notes and 5.875% 2027 Senior Notes are guaranteed on a senior unsecured basis by our material direct and indirect wholly owned domestic subsidiaries and, therefore, are structurally senior to any of our existing and future indebtedness that is not guaranteed by such guarantors and are structurally subordinated to all existing and future indebtedness and other liabilities of our subsidiaries that do not guarantee the 5.75% 2025 Senior Notes and 5.875% 2027 Senior Notes. We may redeem the 5.75% 2025 Senior Notes, in whole or in part, on or after March 1, 2020, at the redemption prices set forth in the indenture, plus accrued and unpaid interest, if any, to, but not including, the date of redemption. We may redeem the 5.875% 2027 Senior Notes, in whole or in part, on or after June 1, 2022, at the redemption prices set forth in the indenture, plus accrued and unpaid interest, if any, to, but not including, the date of redemption, and prior to June 1, 2022, at a redemption price equal to 100% of the principal amount thereof plus a “make-whole” premium set forth in the indenture, plus accrued and unpaid interest, if any, to, but not including, the date of redemption. We may also redeem up to 35% of the aggregate principal amount of the 5.875% 2027 Senior Notes on or prior to June 1, 2022 at a redemption price equal to 105.875% of the principal amount thereof, plus accrued and unpaid interest, if any, to, but not including, the date of redemption with the net cash proceeds of one or more equity offerings. The 6.25% 2040 Senior Notes may be redeemed any time at our option upon not less than 30, nor more than 60, days' prior notice is sent to the holders. The 6.25% 2040 Senior Notes are redeemable at a redemption price equal to the greater of (1) 100% of the principal amount of the notes to be redeemed or (2) the sum of the present values of the remaining scheduled payments of principal and interest on the notes to be redeemed, discounted to the redemption date on a semi-annual basis at the treasury rate plus 40 basis points, plus accrued and unpaid interest, if any, to, but not including, the date of redemption. In addition, if a change of control triggering event, as defined in the applicable indenture, occurs with respect to the unsecured notes, we will be required to offer to purchase the notes of the applicable series at a purchase price equal to 101% of the principal amount, plus accrued and unpaid interest, if any, to, but not including, the date of purchase. The terms of the unsecured notes contain certain customary covenants; however, there are no financial covenants. ABL Facility On March 13, 2020, in connection with the AK Steel Merger, we entered into a new ABL Facility with various financial institutions to replace and refinance Cliffs’ Former ABL Facility and AK Steel Corporation’s former revolving credit facility. The ABL Facility will mature upon the earlier of March 13, 2025 or 91 days prior to the maturity of certain other material debt and provided for up to $2 billion in borrowings, including a $555 million sublimit for the issuance of letters of credit and a $125 million sublimit for swingline loans. Availability under the ABL Facility is limited to an eligible borrowing base, as applicable, determined by applying customary advance rates to eligible accounts receivable, inventory and certain mobile equipment. On March 27, 2020, the ABL Facility was amended, by and among Cliffs, the lenders and the administrative agent. The amendment modified the ABL Facility to, among other things, provide for a new FILO tranche B of commitments in the aggregate amount of $150 million by exchanging existing commitments under the ABL Facility. The total commitments under the ABL Facility after giving effect to the amendment remained at $2 billion. The terms and conditions (other than the pricing) that apply to the FILO tranche are substantially the same as the terms and conditions that apply to the tranche A facility of the ABL Facility immediately prior to the amendment. On December 9, 2020, we entered into the ABL Amendment with various financial institutions. The ABL Amendment modified the ABL Facility to, among other things, increase the amount of tranche A revolver commitments available thereunder by an additional $1.5 billion and increase certain dollar baskets related to certain negative covenants that apply to the ABL Facility. After giving effect to the ABL Amendment, the aggregate principal amount of tranche A revolver commitments under the ABL Facility is $3.35 billion and the aggregate principal amount of FILO tranche B revolver commitments under the ABL Facility remains at $150 million. The ABL Facility and certain bank products and hedge obligations are guaranteed by certain of our existing wholly owned U.S. subsidiaries and are required to be guaranteed by certain of our future U.S. subsidiaries. Amounts outstanding under the ABL Facility are secured by (i) a first-priority security interest in the accounts receivable and other rights to payment, inventory, as-extracted collateral, certain investment property, deposit accounts, securities accounts, certain general intangibles and commercial tort claims, certain mobile equipment, commodities accounts and other related assets of ours, the other borrowers and the guarantors, and proceeds and products of each of the foregoing (collectively, the “ABL Collateral”) and (ii) a second-priority security interest in substantially all of our assets and the assets of the other borrowers and the guarantors other than the ABL Collateral. Borrowings under the ABL Facility bear interest, at our option, at a base rate or, if certain conditions are met, a LIBOR rate, in each case plus an applicable margin. We may amend our ABL Facility to replace the LIBOR rate with one or more secured overnight financing based rates or an alternative benchmark rate, giving consideration to any evolving or then-existing convention for similar dollar denominated syndicated credit facilities for such alternative benchmarks. The ABL Facility contains customary representations and warranties and affirmative and negative covenants including, among others, covenants regarding the maintenance of certain financial ratios if certain conditions are triggered, covenants relating to financial reporting, covenants relating to the payment of dividends on, or purchase or redemption of, our capital stock, covenants relating to the incurrence or prepayment of certain debt, covenants relating to the incurrence of liens or encumbrances, covenants relating to compliance with laws, covenants relating to transactions with affiliates, covenants relating to mergers and sales of all or substantially all of our assets and limitations on changes in the nature of our business. The ABL Facility provides for customary events of default, including, among other things, the event of nonpayment of principal, interest, fees or other amounts, a representation or warranty proving to have been materially incorrect when made, failure to perform or observe certain covenants within a specified period of time, a cross-default to certain material indebtedness, the bankruptcy or insolvency of the Company and certain of its subsidiaries, monetary judgment defaults of a specified amount, invalidity of any loan documentation, a change of control of the Company, and ERISA defaults resulting in liability of a specified amount. If an event of default exists (beyond any applicable grace or cure period), the administrative agent may, and at the direction of the requisite number of lenders shall, declare all amounts owing under the ABL Facility immediately due and payable, terminate such lenders’ commitments to make loans under the ABL Facility and/or exercise any and all remedies and other rights under the ABL Facility. For certain events of default related to insolvency and receivership, the commitments of the lenders will be automatically terminated and all outstanding loans and other amounts will become immediately due and payable. As of December 31, 2020 and 2019, we were in compliance with the ABL Facility liquidity requirements and, therefore, the springing financial covenant requiring a minimum fixed charge coverage ratio of 1.0 to 1.0 was not applicable. The following represents a summary of our borrowing capacity under the ABL Facility: (In Millions) December 31, Available borrowing base on ABL Facility 1 $ 3,500 Borrowings (1,510) Letter of credit obligations 2 (247) Borrowing capacity available $ 1,743 1 As of December 31, 2020, the ABL Facility has a maximum borrowing base of $3.5 billion. The available borrowing base is determined by applying customary advance rates to eligible accounts receivable, inventory and certain mobile equipment. 2 We issued standby letters of credit with certain financial institutions in order to support business obligations including, but not limited to, workers' compensation, employee severance, insurance, operating agreements, IRBs and environmental obligations. Other

FAIR VALUE OF FINANCIAL INSTRUM

FAIR VALUE OF FINANCIAL INSTRUMENTS	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
FAIR VALUE OF FINANCIAL INSTRUMENTS	There were no significant assets or liabilities measured at fair value as of December 31, 2020. The following represents the assets and liabilities measured at fair value as of December 31, 2019: (In Millions) December 31, 2019 Quoted Prices in Active Significant Other Observable Inputs Significant Unobservable Inputs Total Assets: Cash equivalents - Commercial paper $ — $ 188 $ — $ 188 Other current assets: Customer supply agreement — — 45 45 Total $ — $ 188 $ 45 $ 233 The valuation of financial assets classified in Level 2 was determined using a market approach based upon quoted prices for similar assets in active markets, or other inputs that were observable. Our supply agreement with ArcelorMittal USA contained provisions for supplemental revenue or refunds based on the HRC price in the year the iron ore product was consumed in ArcelorMittal USA’s blast furnaces. We accounted for these provisions as derivative instruments at the time of sale and adjusted the derivative instruments to fair value through Revenues each reporting period until the product was consumed and the amounts were settled. These instruments were classified as Level 3 assets. Upon the completion of the AM USA Transaction, the outstanding derivatives were settled as part of acquisition accounting. The following tables represent a reconciliation of the changes in fair value of financial instruments measured at fair value on a recurring basis using significant unobservable inputs (Level 3): (In Millions) Level 3 Assets 2020 2019 Beginning balance - January 1 $ 45 $ 89 Total gains included in earnings 122 79 Settlements (27) (123) Settlement of pre-existing relationship (140) — Ending balance - December 31 $ — $ 45 Total gains for the period included in earnings attributable to the change in unrealized gains on assets still held at the reporting date $ — $ 45 The carrying values of certain financial instruments (e.g. Accounts receivable, net , Accounts payable and Other current liabilities ) approximate fair value and, therefore, have been excluded from the table below. A summary of the carrying value and fair value of other financial instruments were as follows: (In Millions) December 31, 2020 December 31, 2019 Classification Carrying Fair Value Carrying Fair Value Senior notes Level 1 $ 3,802 $ 4,446 $ 2,114 $ 2,237 IRBs due 2024 to 2028 Level 1 94 91 — — EDC Revolving Facility - outstanding balance Level 2 18 18 — — ABL Facility - outstanding balance Level 2 1,510 1,510 — — Total $ 5,424 $ 6,065 $ 2,114 $ 2,237

PENSIONS AND OTHER POSTRETIREME

PENSIONS AND OTHER POSTRETIREMENT BENEFITS	12 Months Ended
	Dec. 31, 2020
Postemployment Benefits [Abstract]
PENSION AND OTHER POSTRETIREMENT BENEFITS	We offer defined benefit pension plans, defined contribution pension plans and OPEB plans to a significant portion of our employees and retirees. Benefits are also provided through multiemployer plans for certain union members. As a result of the acquisitions of AK Steel and ArcelorMittal USA, we assumed the obligations under their defined benefit pension plans, OPEB plans, defined contribution plans and commitments to multiemployer pension plans according to collective bargaining agreements that cover certain union-represented employees. The AK Steel defined benefit pension plans and OPEB plans acquired amounted to a benefit obligation, net of assets of $949 million based on a March 13, 2020 measurement. The ArcelorMittal USA defined benefit pension plans and OPEB plans acquired amounted to a benefit obligation, net of assets of $3,294 million based on a December 9, 2020 measurement. Defined Benefit Pension Plans The defined benefit pension plans are largely noncontributory and limited in participation. Most plans are closed to new participants with only the legacy iron ore hourly and salaried plans still open. The pension benefit calculations vary by plan, but are generally based on employee's years of service and compensation or a fixed rate and years of service. Certain salaried plans calculate benefits using a cash balance formula, which earns interest credits and allocations based on a percent of pay. OPEB Plans We offer postretirement health care and life insurance benefits to retirees through various plans. The vast majority of our plans are closed to new participants. In lieu of retiree medical coverage, many union-represented employees receive a 401(k) contribution per hour worked to a restricted Retiree Health Care Account. Cost sharing features between the employer and retiree vary by plan and several plans include employer caps. Retiree healthcare coverage is provided through programs administered by insurance companies whose charges are based on benefits paid. Certain labor agreements require the funding of VEBAs, which, depending on funding levels, may be used to reimburse the employer for paid benefits. Obligations and Funded Status The following tables and information provide additional disclosures: (In Millions) Pension Benefits OPEB Change in benefit obligations: 2020 2019 2020 2019 Benefit obligations — beginning of year $ 1,021 $ 906 $ 255 $ 242 Service cost 23 17 8 2 Interest cost 64 35 19 10 Actuarial loss 162 112 14 19 Benefits paid (146) (62) (89) (26) Participant contributions — — 22 6 Acquired through business combinations 5,535 — 3,528 — Effect of settlement (94) — — — Other — 13 — 2 Benefit obligations — end of year $ 6,565 $ 1,021 $ 3,757 $ 255 Change in plan assets: Fair value of plan assets — beginning of year $ 749 $ 687 $ 260 $ 240 Actual return on plan assets 472 98 45 35 Participant contributions — — 17 1 Employer contributions 50 16 30 3 Benefits paid (146) (62) (88) (19) Acquired through business combinations 4,301 — 519 — Effect of settlement (94) — — — Other — 10 — — Fair value of plan assets — end of year $ 5,332 $ 749 $ 783 $ 260 Funded status $ (1,233) $ (272) $ (2,974) $ 5 Amounts recognized in Statements of Financial Position: Non-current assets $ 3 $ — $ 54 $ 49 Current liabilities (12) — (139) (4) Non-current liabilities (1,224) (272) (2,889) (40) Total amount recognized $ (1,233) $ (272) $ (2,974) $ 5 Amounts recognized in accumulated other comprehensive loss: Net actuarial loss $ 164 $ 382 $ 56 $ 73 Prior service cost (credit) 6 7 (6) (8) Net amount recognized $ 170 $ 389 $ 50 $ 65 The accumulated benefit obligation for all defined benefit pension plans was $6,537 million and $1,010 million at December 31, 2020 and 2019, respectively. Components of Net Periodic Benefit Cost (Credit) (In Millions) Pension Benefits OPEB 2020 2019 2018 2020 2019 2018 Service cost $ 23 $ 17 $ 19 $ 8 $ 2 $ 2 Interest cost 64 35 30 19 10 8 Expected return on plan assets (140) (55) (60) (20) (17) (18) Amortization: Net actuarial loss 27 24 21 3 5 5 Prior service costs (credits) 1 1 2 (2) (2) (3) Settlements (6) — 1 — — — Net periodic benefit cost (credit) $ (31) $ 22 $ 13 $ 8 $ (2) $ (6) For 2021, we estimate net periodic benefit cost (credit) as follows: (In Millions) Defined benefit pension plans $ (168) OPEB plans 86 Total $ (82) Components of Accumulated Other Comprehensive Loss (Income) The following includes details on the significant actuarial losses (gains) impacting the benefit obligation: (In Millions) Pension Benefits OPEB 2020 2019 2020 2019 Discount rates $ 181 $ 106 $ 44 $ 26 Demographic (gains) losses (3) 12 (11) 4 Mortality (16) (6) (4) (4) Per capita claims — — (10) (9) Other — — (5) 3 Actuarial loss on benefit obligation 162 112 14 20 Actual returns on assets over expected (332) (44) (26) (18) Amortization of net actuarial loss (27) (24) (3) (5) Amortization of prior service credits (costs) (1) (1) 2 2 Settlements 6 — — — Other (27) 7 (2) (5) Total recognized in accumulated other comprehensive loss (income) $ (219) $ 50 $ (15) $ (6) Contributions Annual contributions to the pension plans are made within income tax deductibility restrictions in accordance with statutory regulations. OPEB plans are not subject to minimum regulatory funding requirements, but rather amounts are contributed pursuant to bargaining agreements. (In Millions) Pension Other Benefits Company Contributions (Reimbursements) VEBA Direct Total 2019 $ 16 $ — $ 4 $ 4 2020 50 — 25 25 2021 (Expected) 1 202 (16) 144 128 1 Pursuant to the applicable bargaining agreements, benefits can be paid from certain VEBAs that are at least 70% funded (all VEBAs are over 70% funded at December 31, 2020). Certain agreements with plans holding VEBA assets have capped healthcare costs. For the ArcelorMittal USA VEBA, depending on funding levels and/or Company profits, we may withdraw money from the VEBA plans to the extent funds are available for costs in excess of the cap. The 2021 expected pension contributions include $118 million in deferred 2020 pension contributions in connection with the CARES Act that were paid on January 4, 2021. Estimated Future Benefit Payments (In Millions) Pension OPEB 2021 $ 486 $ 191 2022 462 185 2023 480 180 2024 455 178 2025 433 176 2026-2030 1,983 884 Assumptions The discount rates used to measure plan liabilities as of the December 31 measurement date are determined individually for each plan. The discount rates are determined by matching the projected cash flows used to determine the plan liabilities to a projected yield curve of high-quality corporate bonds available at the measurement date. Discount rates for expense are calculated using the granular approach for each plan. Depending on the plan, we use either company-specific base mortality tables or tables issued by the Society of Actuaries. We adopted the Pri-2012 mortality tables from the Society of Actuaries in 2019. On December 31, 2020, the assumed mortality improvement projection was updated from generational scale MP-2019 to generational scale MP-2020 for the Pri-2012 mortality tables. The following represents weighted-average assumptions used to determine benefit obligations: Pension Benefits OPEB December 31, December 31, 2020 2019 2020 2019 Discount rate 2.34 % 3.27 % 2.71 % 3.28 % Interest crediting rate 5.25 6.00 N/A N/A Compensation rate increase 2.56 2.53 3.00 3.00 The following represents weighted-average assumptions used to determine net benefit cost: Pension Benefits OPEB December 31, December 31, 2020 2019 2018 2020 2019 2018 Obligation discount rate 3.02 % 4.27 % 3.58 % 3.28 % 4.29 % 3.60 % Service cost discount rate 3.34 4.35 3.64 3.35 4.49 3.73 Interest cost discount rate 2.53 3.92 3.16 2.51 3.94 3.11 Interest crediting rate 5.50 6.00 6.00 N/A N/A N/A Expected return on plan assets 7.69 8.25 8.25 6.82 7.00 7.00 Compensation rate increase 2.56 2.53 2.49 3.00 3.00 3.00 The following represents assumed weighted-average health care cost trend rates: December 31, 2020 2019 Health care cost trend rate assumed for next year 6.05 % 6.50 % Ultimate health care cost trend rate 4.59 5.00 Year that the ultimate rate is reached 2031 2026 Plan Assets Our financial objectives with respect to our pension and VEBA assets are to fully fund the actuarial accrued liability for each of the plans, to maximize investment returns within reasonable and prudent levels of risk, and to maintain sufficient liquidity to meet benefit obligations on a timely basis. Our investment objective is to outperform the expected return on assets assumption used in the plans’ actuarial reports over the life of the plans. The expected return on assets takes into account historical returns and estimated future long-term returns based on capital market assumptions applied to the asset allocation strategy. The expected return is net of investment expenses paid by the plans. In addition, investment performance is monitored on a quarterly basis by benchmarking to various indices and metrics for the one-, three- and five-year periods. The asset allocation strategy is determined through a detailed analysis of assets and liabilities by plan, which defines the overall risk that is acceptable with regard to the expected level and variability of portfolio returns, surplus (assets compared to liabilities), contributions and pension expense. The asset allocation review process involves simulating capital market behaviors including global asset class performance, inflation and interest rates in order to evaluate various asset allocation scenarios and determine the asset mix with the highest likelihood of meeting financial objectives. The process includes factoring in the current funded status and likely future funded status levels of the plans by taking into account expected growth or decline in the contributions over time. The asset allocation strategy varies by plan. The following table reflects the actual asset allocations for pension and VEBA assets as of December 31, 2020 and 2019, as well as the 2021 weighted average target asset allocations. Equity investments include securities in large-cap, mid-cap and small-cap companies located in the U.S. and worldwide. Fixed income investments primarily include corporate bonds and government debt securities. Pension Assets VEBA Assets Asset Category 2021 Percentage of 2021 Percentage of 2020 2019 2020 2019 Equity securities 41.3 % 51.8 % 44.0 % 20.3 % 22.2 % 7.2 % Fixed income 39.7 33.8 27.6 69.6 66.4 79.8 Hedge funds 5.0 2.2 5.4 1.1 1.8 4.8 Private equity 2.2 2.1 6.6 1.4 0.4 0.7 Structured credit 5.2 5.0 7.0 1.0 0.9 2.1 Real estate 5.2 3.3 9.4 1.1 1.8 5.4 Absolute return fixed income 1.4 1.8 — 5.5 6.5 — Total 100.0 % 100.0 % 100.0 % 100.0 % 100.0 % 100.0 % As a practical expedient, in accordance with ASC 820-10, certain investments that are measured at fair value using the NAV per share have not been classified in the fair value hierarchy below. NAV is based on the value of the underlying assets owned by the fund, minus its liabilities, and then divided by its number of shares outstanding. The fair value of our pension assets by asset category is as follows: (In Millions) Quoted Prices in Active Significant Other Significant Investments Measured at Net Asset Value Total Asset Category 2020 2019 2020 2019 2020 2019 2020 2019 2020 2019 Equity securities: U.S. equities $ 1,163 $ 169 $ — $ — $ — $ — $ 787 $ — $ 1,950 $ 169 Global equities 615 161 — — — — 195 — 810 161 Fixed income: U.S. government securities 1 141 11 295 22 — — 40 — 476 33 U.S. corporate bonds 512 174 466 — — — 303 — 1,281 174 Non U.S. and other bonds — — 46 — — — — — 46 — Hedge funds — — — — 118 40 — — 118 40 Private equity — — — — 114 50 — — 114 50 Structured credit — — — — 264 52 — — 264 52 Real estate — — — — 174 70 — — 174 70 Absolute return fixed income — — — — — — 99 — 99 — Total $ 2,431 $ 515 $ 807 $ 22 $ 670 $ 212 $ 1,424 $ — $ 5,332 $ 749 1 Includes cash equivalents. Assets for OPEB plans include VEBA trusts pursuant to bargaining agreements that are available to fund retired employees’ life insurance obligations and medical benefits. The fair value of our other benefit plan assets by asset category is as follows: (In Millions) Quoted Prices in Active Significant Other Significant Investments Measured at Net Asset Value Total Asset Category 2020 2019 2020 2019 2020 2019 2020 2019 2020 2019 Equity securities: U.S. equities $ 26 $ 12 $ — $ — $ — $ — $ 93 $ — $ 119 $ 12 Global equities 6 7 — — — — 49 — 55 7 Fixed income: U.S. government securities 1 62 — 94 — — — — — 156 — U.S. corporate bonds 237 166 127 41 — — — — 364 207 Hedge funds — — — — 14 12 — — 14 12 Private equity — — — — 3 2 — — 3 2 Structured credit — — — — 7 6 — — 7 6 Real estate — — — — 14 14 — — 14 14 Absolute return fixed income — — — — — — 51 — 51 — Total $ 331 $ 185 $ 221 $ 41 $ 38 $ 34 $ 193 $ — $ 783 $ 260 1 Includes cash equivalents. The following represents the fair value measurements of changes in plan assets using significant unobservable inputs (Level 3): (In Millions) Pension Assets VEBA Assets 2020 2019 2020 2019 Beginning balance — January 1 $ 212 $ 229 $ 34 $ 36 Actual return on plan assets: Relating to assets still held at the reporting date 8 (1) 2 1 Relating to assets sold during the period 6 30 1 — Purchases 195 17 — — Sales (13) (60) (1) (3) Acquired through business combinations 262 — 2 — Other — (3) — — Ending balance — December 31 $ 670 $ 212 $ 38 $ 34 Following is a description of the inputs and valuation methodologies used to measure the fair value of our plan assets. Equity Securities Equity securities classified as Level 1 investments include U.S. large-, small- and mid-cap investments and international equities. These investments are comprised of securities listed on an exchange, market or automated quotation system for which quotations are readily available. The valuation of these securities is determined using a market approach and is based upon unadjusted quoted prices for identical assets in active markets. Fixed Income Fixed income securities classified as Level 1 investments include bonds, government debt securities and cash equivalents. These investments are comprised of securities listed on an exchange, market or automated quotation system for which quotations are readily available. The valuation of these securities is determined using a market approach and is based upon unadjusted quoted prices for identical assets in active markets. Also included in fixed income is a portfolio of U.S. Treasury STRIPS, which are zero-coupon bearing fixed income securities backed by the full faith and credit of the U.S. government. The securities sell at a discount to par because there are no incremental coupon payments. STRIPS are not issued directly by the Treasury, but rather are created by a financial institution, government securities broker or government securities dealer. Liquidity on the issue varies depending on various market conditions; however, in general, the STRIPS market is slightly less liquid than that of the U.S. Treasury Bond market. The STRIPS are priced daily through a bond pricing vendor and are classified as Level 2. Hedge Funds Hedge funds are alternative investments comprised of direct or indirect investment in offshore hedge funds with an investment objective to achieve equity-like returns with one half the volatility of equities and moderate correlation. The valuation techniques used to measure fair value attempt to maximize the use of observable inputs and minimize the use of unobservable inputs. Considerable judgment is required to interpret the factors used to develop estimates of fair value. Valuations of the underlying investment funds are obtained and reviewed. The securities that are valued by the funds are interests in the investment funds and not the underlying holdings of such investment funds. Thus, the inputs used to value the investments in each of the underlying funds may differ from the inputs used to value the underlying holdings of such funds. Hedge funds are valued monthly and recorded on a one-month lag. Private Equity Funds Private equity funds are alternative investments that represent direct or indirect investments in partnerships, venture funds or a diversified pool of private investment vehicles (fund of funds). Investment commitments are made in private equity funds based on an asset allocation strategy, and capital calls are made over the life of the funds to fund the commitments. As of December 31, 2020, remaining commitments total $86 million for our pension and OPEB plans. Committed amounts are funded from plan assets when capital calls are made. Investment commitments are not pre-funded in reserve accounts. Private equity investments are valued quarterly and recorded on a one-quarter lag. For private equity investment values reported on a lag, current market information is reviewed for any material changes in values at the reporting date. Capital distributions for the funds do not occur on a regular frequency. Liquidation of these investments would require sale of the partnership interest. Structured Credit Structured credit funds provide flexibility and access to both complex and illiquid premiums by investing across global, public and private residential, commercial, corporate and specialty credit markets that are priced based on valuations provided by independent, third-party pricing agents, if available. Such values generally reflect the last reported sales price if the security is actively traded. The third-party pricing agents may also value structured credit investments at an evaluated bid price by employing methodologies that utilize actual market transactions, broker-supplied valuations or other methodologies designed to identify the market value of such securities. Structured credit investments are valued monthly and certain funds have an initial lock-up period and withdrawal restrictions on a semi-annual and quarterly basis. For structured credit investment values reported on a lag, current market information is reviewed for any material changes in values at the reporting date. Real Estate The real estate portfolio for the pension plans is an alternative investment comprised of funds with strategic categories of real estate investments. All real estate holdings are appraised externally at least annually, and appraisals are conducted by reputable, independent appraisal firms that are members of the Appraisal Institute. All external appraisals are performed in accordance with the Uniform Standards of Professional Appraisal Practices. The property valuations and assumptions about each property are reviewed quarterly by the investment manager and values are adjusted if there has been a significant change in circumstances relating to the property since the last external appraisal. The fair values of the funds are updated on either a monthly or quarterly basis. Redemption requests are considered on a quarterly basis, subject to notice requirements. The real estate fund of funds investment for the Empire-Tilden, Hibbing and United Taconite VEBA plans invests in pooled investment vehicles that, in turn, invest in commercial real estate properties. Valuations are performed quarterly and financial statements are prepared on a semi-annual basis, with annual audited statements. Asset values for this fund are reported with a one-quarter lag, and current market information is reviewed for any material changes in values at the reporting date. Withdrawals are permitted on the last business day of each quarter subject to a 95-day prior written notice. Absolute Return Fixed Income Absolute return fixed income investments consist of a global fixed income fund with the investment objective of generating positive absolute returns over a full market cycle. The fund's investments in securities, forward exchange contracts and futures contracts are reported at fair value on a recurring monthly basis. The fund's trustee values securities based upon independent pricing sources and futures contracts are valued at closing settled prices. Redemptions of the fund at NAV are permitted monthly under most circumstances. Defined Contribution Plans Most employees are eligible to participate in various defined contribution plans. Certain of these plans have features with matching contributions or other Company contributions based on our financial results. Company contributions to these plans are expensed as incurred. Total expense from these plans was $22 million, $3 million and $3 million in 2020, 2019 and 2018, respectively. Multiemployer Plans We contribute to multiemployer pension plans according to collective bargaining agreements that cover certain union-represented employees. The following risks of participating in these multiemployer plans differ from single employer pension plans: • Employer contributions to a multiemployer plan may be used to provide benefits to employees of other participating employers. • If a participating employer stops contributing to a multiemployer plan, the remaining participating employers may need to assume the unfunded obligations of the plan. • If the multiemployer plan becomes significantly underfunded or is unable to pay its benefits, we may be required to contribute additional amounts in excess of the rate required by the collective bargaining agreements. • If we choose to stop participating in a multiemployer plan, we may be required to pay that plan an amount based on the underfunded status of the plan, referred to as a withdrawal liability. We are a party to five collective bargaining agreements at our Ashland Works, Mansfield Works, United Taconite, Tubular Components and the majority of our ArcelorMittal USA locations that require contributions to the Steelworkers Pension Trust. We are a party to three collective bargaining agreements at Butler Works, Middletown Works and Zanesville Works that require contributions to the International Association of Machinists and Aerospace Workers ("IAM") National Pension Fund's National Pension Plan. The plan voluntarily elected to place itself in the "Red Zone" in April 2019 and has implemented a rehabilitation plan. Additional contributions will be required as part of the rehabilitation plan until the plan exits the critical status. Information with respect to multiemployer plans in which we participate follows: Pension Fund EIN/Pension Plan Number Pension Protection Act Zone Status (a) FIP/RP Status Pending/Implemented (b) Contributions Surcharge Imposed (c) Expiration Date of Collective Bargaining Agreement 2020 2019 2020 2019 2018 Steelworkers Pension Trust 23-6648508/499 Green Green No $ 14 $ 4 $ 4 No 1/22/2021 to 10/1/2022 IAM National Pension Fund’s National Pension Plan 51-6031295/002 Red Green Yes 16 — — Yes 5/31/2022 to 5/15/2023 American Maritime Officers Plan 13-1936709/001 Green Green No — — — No 7/31/2021 Total $ 30 $ 4 $ 4 (a) The most recent Pension Protection Act zone status available in 2020 and 2019 is for each plan's year-end at December 31, 2019 and 2018. The plan's actuary certifies the zone status. Generally, plans in the red zone are less than 65% funded, plans in the yellow zone are between 65% and 80% funded, and plans in the green zone are at least 80% funded. (b) The "FIP/RP Status Pending/Implemented" column indicates plans for which a financial improvement plan or a rehabilitation plan is either pending or has been implemented, as defined by ERISA. (c) The surcharge represents an additional required contribution due as a result of the critical funding status of the plan. Prior to the Acquisitions, AK Steel and ArcelorMittal USA made up over 30% of the contributions to the Steelworkers Pension Trust in the last three years. Only two other employers contributed more than 5% during this period. As of December 31, 2019 (the last date for which we have information), the Steelworkers Pension Trust had a total actuarial liability of $5,748 million and assets with a market value of $5,372 million, for a funded ratio of about 93%.

STOCK COMPENSATION PLANS

STOCK COMPENSATION PLANS	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
STOCK COMPENSATION PLANS	At December 31, 2020, we had outstanding awards under two share-based compensation plans: the A&R 2015 Equity Plan and the 2012 Amended Equity Plan. On March 13, 2020, the maximum number of shares that may be issued under the A&R 2015 Equity Plan increased by 5.7 million common shares in relation to the outstanding AK Steel stock-based incentive awards that we converted at a 0.400 rate of exchange. The converted stock-based incentive awards include 2.0 million stock options, 1.0 million long-term performance plan awards, 0.5 million performance shares, 0.2 million restricted stock awards and 0.3 million restricted stock units. As of December 31, 2020, there were 4.9 million remaining shares available for grant under the A&R 2015 Equity Plan. No additional grants were issued from the 2012 Amended Equity Plan after the date of approval of the A&R 2015 Equity Plan; however, all awards previously granted under the 2012 Amended Equity Plan will continue in accordance with the terms of the outstanding awards. Stock-Based Compensation Expense The following table summarizes the total compensation expense recognized for stock-based compensation awards: (In Millions, except per Year Ended December 31, 2020 2019 2018 Cost of goods sold $ (2) $ (2) $ (2) Selling, general and administrative expenses (13) (16) (13) Acquisition-related costs (2) — — Stock based compensation expense (17) (18) (15) Income tax benefit 1 4 4 — Stock based compensation expense, net of tax $ (13) $ (14) $ (15) Decrease in basic earnings per common share $ (0.03) $ (0.05) $ (0.05) Decrease in diluted earnings per common share $ (0.03) $ (0.05) $ (0.05) 1 No income tax benefit in 2018 due to the full valuation allowance. The total compensation cost related to outstanding awards not yet recognized is $14 million at December 31, 2020. This expense is expected to be recognized over the remaining weighted-average period of approximately 1.7 years. Performance Shares The following table summarizes the performance award activity: Year Ended December 31, 2020 2019 2018 Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Outstanding at beginning of year 1,935,878 $ 15.58 1,424,723 $ 14.46 1,848,312 $ 13.42 Granted 2,510,853 5.49 572,104 18.31 675,599 11.93 Distributed (1,938,786) 12.23 — — (489,953) 10.89 Performance adjustment 549,154 15.63 — — (560,720) 10.69 Forfeited/canceled (604,873) 5.70 (60,949) 15.12 (48,515) 19.33 Outstanding at end of year 2,452,226 $ 10.34 1,935,878 $ 15.58 1,424,723 $ 14.46 The 2.5 million awards granted in 2020 include 1.0 million long-term performance plan awards and 0.5 million performance shares relating to the AK Steel replacement awards. As of December 31, 2020, 0.3 million long-term performance plan awards and 0.1 million performance shares were outstanding as a result of qualifying termination events that triggered accelerated performance share payouts and prorated long-term performance plan awards payouts at target. The long-term performance plan awards are based on a three-year Adjusted EBITDA metric. The weighted average grant date fair value for the converted performance awards was $4.59 per share. The outstanding performance shares vest over a period of three years and are intended to be paid out in common shares. Performance is measured on the basis of relative TSR for the period and measured against the constituents of the S&P Metals and Mining ETF Index. The number of shares actually earned at the end of the three-year period will vary, based on performance, from 0% to 200% of the number of performance shares granted. Restricted Stock Units The following table summarizes the restricted stock units activity: Year Ended December 31, 2020 2019 2018 Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Outstanding at beginning of year 2,147,183 $ 9.10 4,694,360 $ 4.18 4,403,112 $ 3.64 Granted 1,160,928 4.87 572,104 11.24 685,599 7.53 Distributed (1,101,115) 8.58 (3,058,307) 1.95 (254,196) 3.30 Forfeited/canceled (63,413) 7.31 (60,974) 9.31 (140,155) 5.17 Outstanding at end of year 2,143,583 $ 7.12 2,147,183 $ 9.10 4,694,360 $ 4.18 The 1.2 million restricted stock units granted in 2020 include 0.2 million restricted stock awards relating to AK Steel replacement awards. The restricted stock awards relating to AK Steel vest ratably on the first, second and third anniversaries of the grant. We valued the restricted stock awards and restricted stock units at $4.87 per share using the closing price of our common shares on March 13, 2020, the grant date. All of the outstanding restricted stock units are subject to continued employment, are retention based, and are payable in common shares or cash in certain circumstances at a time determined by the Compensation Committee at its discretion. The restricted stock units that were granted in 2020, 2019 and 2018 cliff vest over three years on December 31, 2022, December 31, 2021 and December 31, 2020, respectively. Stock Options The following table summarizes the stock option activity: Year Ended December 31, 2020 2019 2018 Number of Shares Weighted Average Exercise Price Number of Shares Weighted Average Exercise Price Number of Shares Weighted Average Excercise Price Outstanding at beginning of year 563,230 $ 10.42 563,230 $ 10.42 599,870 $ 10.25 Granted 2,010,277 11.86 — — — — Exercised (79,973) 7.01 — — (36,640) 7.70 Forfeited/canceled (7,726) 41.04 — — — — Outstanding at end of year 2,485,808 $ 11.60 563,230 $ 10.42 563,230 $ 10.42 Exercisable at end of year 2,172,052 $ 11.86 563,230 $ 10.42 563,230 $ 10.42 Stock options granted to date generally vest over a period from one to three years with an expiration date at ten years from the date of grant. On March 13, 2020, we granted 2.0 million options as AK Steel replacement awards. The weighted average fair value of the converted options was $0.51 per share and was calculated using the Black-Scholes option-pricing model. Qualifying termination events resulted in vest date accelerations and reductions to the option expiration date from ten years to three years. The total intrinsic value of options exercised in 2020 and 2018 were immaterial. For options outstanding at December 31, 2020, the weighted-average remaining contractual life was 3.2 years and the aggregate intrinsic value was $11 million. For options exercisable at December 31, 2020, the weighted-average remaining contractual life was 2.5 years and the aggregate intrinsic value was $9 million. Nonemployee Directors Our nonemployee directors are entitled to receive restricted share awards under the Directors’ Plan. For 2020, 2019 and 2018, nonemployee directors were granted a specified number of restricted shares, with a value equal to $100,000. The number of shares is based on the closing price of our common shares on the date of the Annual Meeting. The restricted share awards issued under the Directors' Plan generally vest 12 months from the grant date. The awards are subject to any deferral election and the terms of the Directors’ Plan and an award agreement. On March 13, 2020, 0.3 million restricted stock units previously awarded to the members of the AK Steel board of directors were distributed per the terms of the AK Steel Merger Agreement. For the last three years, grants of restricted and/or deferred shares have been awarded to elected or re-elected nonemployee directors as follows: Year of Grant Restricted Shares Deferred Shares 2020 253,809 54,794 2019 86,477 23,659 2018 92,718 17,170

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income (loss) from continuing operations before income taxes includes the following components: (In Millions) 2020 2019 2018 United States $ (201) $ 313 $ 565 Foreign 8 — — $ (193) $ 313 $ 565 The components of the income tax provision (benefit) on continuing operations consist of the following: (In Millions) 2020 2019 2018 Current provision (benefit): United States federal $ (2) $ (1) $ (1) United States state & local — — — Foreign (1) — 1 (3) (1) — Deferred provision (benefit): United States federal (95) 19 (475) United States state & local (11) — — Foreign (2) — — Total income tax provision (benefit) from continuing operations $ (111) $ 18 $ (475) Reconciliation of our income tax attributable to continuing operations computed at the U.S. federal statutory rate is as follows: (In Millions) 2020 2019 2018 Tax at U.S. statutory rate $ (41) 21 % $ 66 21 % $ 119 21 % Increase (decrease) due to: Percentage depletion in excess of cost depletion (42) 22 (49) (16) (55) (10) Non-taxable income related to noncontrolling interests (9) 4 — — — — Luxembourg legal entity reduction — — 846 271 162 29 Valuation allowance release: Current year activity — — — — (80) (14) Release of U.S. valuation allowance — — — — (461) (81) Luxembourg legal entity reduction — — (846) (271) (162) (29) State taxes, net (11) 6 — — — — Other items, net (8) 4 1 1 2 — Provision for income tax expense (benefit) and effective income tax rate including discrete items $ (111) 57 % $ 18 6 % $ (475) (84) % The increase in income tax benefit from 2019 to 2020 is directly correlated to the increase in pre-tax book loss from the prior period for both federal and state income tax purposes. The other primary driver of the increase in income tax benefit is related to income of noncontrolling interests for which no tax is recognized. The increase in income tax expense from 2018 to 2019 is primarily due to release of the valuation allowance in the U.S. of $461 million in 2018, which did not recur in 2019. The Luxembourg legal entity reduction relates to initiatives resulting in the dissolution of certain entities and settlement of related financial instruments in the years ended December 31, 2019 and 2018. These 2019 and 2018 NOL deferred tax asset reductions resulted in tax expense of $846 million and $162 million, respectively, which were fully offset by decreases in the respective valuation allowance. The components of income taxes for other than continuing operations consisted of the following: (In Millions) 2020 2019 2018 Other comprehensive income: Postretirement benefit liability $ (52) $ 11 $ 4 Unrealized net loss on derivative financial instruments (1) — 1 Total $ (53) $ 11 $ 5 Significant components of our deferred tax assets and liabilities are as follows: (In Millions) 2020 2019 Deferred tax assets: Operating loss and other carryforwards $ 1,236 $ 795 Pension and OPEB liabilities 228 114 Property, plant and equipment and mineral rights — 1 State and local 132 71 Other liabilities 190 70 Total deferred tax assets before valuation allowance 1,786 1,051 Deferred tax asset valuation allowance (836) (441) Net deferred tax assets 950 610 Deferred tax liabilities: Investment in partnerships (144) (137) Property, plant and equipment and mineral rights (246) — Other assets (68) (14) Total deferred tax liabilities (458) (151) Net deferred tax assets $ 492 $ 459 We had gross domestic (including states) and foreign NOLs of $7,444 million and $1,592 million, respectively, at December 31, 2020. We had gross domestic and foreign NOLs of $3,459 million and $1,592 million, respectively, at December 31, 2019. The U.S. federal NOLs will begin to expire in 2034 and state NOLs will begin to expire in 2021. The foreign NOLs can be carried forward indefinitely. We had foreign tax credit carryforwards of $6 million at December 31, 2019, which expired in 2020. We had gross interest expense limitation carryforwards of $80 million and $55 million for the years ended December 31, 2020 and 2019, respectively. This interest expense can be carried forward indefinitely. The changes in the valuation allowance are presented below: (In Millions) 2020 2019 2018 Balance at beginning of year $ 441 $ 1,287 $ 1,983 Change in valuation allowance: Included in income tax expense (benefit) (3) (846) (691) Change in deferred assets in other comprehensive income — — (5) Increase from acquisitions 398 — — Balance at end of year $ 836 $ 441 $ 1,287 During 2020, we recorded a $398 million valuation allowance through opening balance sheet adjustments to reflect the portion of federal and state NOLs that are limited under Section 382 of the IRC acquired through the AK Steel Merger. During 2019, a legal entity reduction initiative was completed in Luxembourg resulting in the dissolution of certain entities and settlement of related financial instruments, triggering the utilization of $1.3 billion of NOL deferred tax asset and reversal of the intercompany notes deferred tax liability of $447 million. In addition, prior year adjustments in Luxembourg and a statutory rate reduction from 26.01% to 24.94% resulted in a net increase to the operating loss carryforward deferred tax asset of $46 million. The total net deferred tax reduction resulted in an expense of $846 million which was fully offset by a decrease in the valuation allowance. During 2018, a similar legal entity reduction initiative was completed resulting in the dissolution of certain Luxembourg entities which resulted in a decrease in the NOL deferred tax asset of $162 million which was fully offset by a decrease in valuation allowance. We continue to maintain a full valuation allowance against the remaining Luxembourg net deferred tax assets of $397 million at December 31, 2020 and 2019. Our losses in Luxembourg in recent periods represent sufficient negative evidence to require a full valuation allowance against the deferred tax assets in that jurisdiction. We intend to maintain a valuation allowance against the deferred tax assets related to these operating losses, until sufficient positive evidence exists to support the realization of such assets. We recorded a $696 million net decrease in the valuation allowance in the year ended December 31, 2018. As of December 31, 2018, our U.S. operations emerged from a three-year cumulative loss position. As the significant negative evidence of cumulative losses had been eliminated, we undertook an evaluation of the continuing need for a valuation allowance on the U.S. deferred tax assets, the majority of which related to the U.S. tax NOLs. In completing our evaluation of whether a valuation allowance was still needed, we considered all available positive and negative evidence. Positive evidence considered included the emergence from the three-year cumulative loss position, our long-term customer contracts with minimum tonnage requirements, the global scarcity of iron ore pellets, near term forecasts of strong profitability and the recently revised IRC Section 163(j) interest deduction limitation. Negative evidence included the overall size of the deferred tax asset with limited carryforward and no carryback opportunity, the finite nature of the iron ore resources we mine, the uncertainty of steel tariffs that positively impacted our revenue rates in 2018 and the various market signs that the U.S. economy may be nearing the end of the current expansion. We also considered that future realization of the deferred tax assets depends on the existence of sufficient taxable income of the appropriate character during the carryforward period. In considering sources of taxable income, we identified that a portion of the deferred tax assets would be utilized by existing taxable temporary differences reversing in the same periods as existing deductible temporary differences. In addition, we determined that carryback opportunities and tax planning strategies do not exist as potential sources of future taxable income. Lastly, forecasting future taxable income was considered, but was challenging in a cyclical industry such as ours as it relies heavily on the accuracy of key assumptions, particularly about key pricing benchmarks. Because historical information is verifiable and more objective than forecast information and due to the cyclicality of the industry, we developed an estimate of future income based on our historical earnings through the most recent industry cycle. We adjusted historical earnings for certain non-recurring items as well as to reflect the current corporate structure by eliminating the impact of discontinued operations and extinguished debt (“core earnings”). Additionally, we adjusted core earnings to reflect the impact of the recently revised IRC Section 163(j) interest expense deduction limitation as well as permanent tax adjustments. The IRC Section 163(j) limitation would limit our interest expense deduction, particularly in down years in the industry cycle, resulting in higher taxable income. Based on the core earnings analysis, our average annual book taxable income through the business cycle was in excess of the estimated $10 million taxable income that would be required annually to fully utilize the deferred tax assets within the 19-year carryforward period. We ascribed significant weight in our assessment to the core earnings analysis and the resulting projection of taxable income through the industry cycle. Based on the weight of this positive evidence, and after considering the other available positive and negative evidence, we determined that it was appropriate to release all of the valuation allowance related to U.S. federal deferred tax assets at December 31, 2018 as it was more likely than not that the entire amount of the U.S. deferred tax asset would be realized before the end of the carryforward period. The income tax benefit recorded for the reversal of the valuation allowance against the U.S. deferred tax assets was $461 million for the year ended December 31, 2018. We also have a valuation allowance recorded against certain state NOLs and foreign tax credits, which are expected to expire before utilization. At December 31, 2020 and 2019, we had a valuation allowance recorded against certain state NOLs of $98 million and $38 million, respectively. At December 31, 2019, we had a valuation allowance recorded against certain foreign tax credits of $6 million, which expired in 2020. At December 31, 2020 and 2019, we had no cumulative undistributed earnings of foreign subsidiaries included in retained earnings. Accordingly, no provision has been made for U.S. deferred taxes related to future repatriation of earnings. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: (In Millions) 2020 2019 2018 Unrecognized tax benefits balance as of January 1 $ 29 $ 29 $ 33 Increase for tax positions in current year 7 — 4 Decrease for tax positions of prior year (4) — — Lapses in statutes of limitations — — (8) Increases from acquisitions 75 — — Unrecognized tax benefits balance as of December 31 $ 107 $ 29 $ 29 At December 31, 2020 and 2019, we had $107 million and $29 million, respectively, of unrecognized tax benefits recorded. Of this amount, $9 million was recorded in Other current liabilities for the year ended December 31, 2020. Additionally, $2 million and $4 million, were recorded in Other non-current liabilities for the years ended December 31, 2020 and 2019, respectively, and $96 million was recorded in Other non-current assets for both years in the Statements of Consolidated Financial Position. If the unrecognized tax benefits were recognized, the entire $11 million would impact the effective tax rate. We do not expect that the amount of unrecognized benefits will change significantly within the next 12 months. At December 31, 2020 and 2019, we had $1 million and $4 million, respectively, of accrued interest and penalties related to the unrecognized tax benefits recorded in Other non-current liabilities in the Statements of Consolidated Financial Position. Tax years 2016 and forward remain subject to examination for the U.S., and tax years 2008 and forward remain subject to examination for Canada.

LEASE OBLIGATIONS

LEASE OBLIGATIONS	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
LEASE OBLIGATIONS	Our operating leases consist primarily of leases for office space, iron ore vessels, rail cars, processing equipment and mining equipment. Our finance leases consist primarily of processing equipment and mining equipment. We use our incremental borrowing rate as the discount rate to determine the present value of the lease payments, as our leases do not have readily determinable implicit discount rates. Our incremental borrowing rate is the rate of interest that we would have to borrow on a collateralized basis over a similar term and amount in a similar economic environment to pay our lease obligations. We determine the incremental borrowing rates for our leases by adjusting the local risk-free interest rate with a credit risk premium corresponding to our credit rating. From time to time, we may enter into arrangements for the construction or purchase of an asset and then enter into a financing arrangement to lease the asset. We recognize leased assets and liabilities under these arrangements when we obtain control of the asset. Lease costs are presented below: (In Millions) Year Ended December 31, 2020 2019 Operating leases $ 43 $ 4 Finance leases: Amortization of lease cost 15 6 Interest on lease liabilities 4 2 Short-term leases 13 6 Total $ 75 $ 18 Other information related to leases was as follows: (In Millions) Year Ended December 31, 2020 2019 Cash paid for amounts included in measurement of lease liabilities: Operating leases within cash flows from operating activities $ 43 $ 4 Finance leases within cash flows from operating activities $ 4 $ 2 Finance leases within cash flows from financing activities $ 15 $ 6 Right-of-use assets obtained in exchange for new finance lease liabilities 1 $ 44 $ 29 Weighted-average remaining lease term - operating leases (in years) 8 10 Weighted-average remaining lease term - finance leases (in years) 5 6 Weighted-average discount rate - operating leases 8 % 8 % Weighted-average discount rate - finance leases 4 % 5 % 1 Right-of-use assets obtained in the Acquisitions are not included in this figure. Future minimum lease payments under noncancellable finance and operating leases as of December 31, 2020 were as follows: (In Millions) Finance Operating Leases 2021 $ 100 $ 70 2022 95 53 2023 78 46 2024 23 38 2025 22 34 Thereafter 76 122 Total future minimum lease payments 394 363 Less: imputed interest 59 99 Total lease payments 335 264 Less: current portion of lease liabilities 91 53 Long-term lease liabilities $ 244 $ 211 The current and long-term portions of our finance lease liabilities are included in Other current liabilities and Other non-current liabilities , respectively. The current and long-term portions of our operating lease liabilities are included in Other current liabilities and Other non-current liabilities , respectively.

ASSET RETIREMENT OBLIGATIONS

ASSET RETIREMENT OBLIGATIONS	12 Months Ended
	Dec. 31, 2020
Environmental Remediation Obligations [Abstract]
Asset Retirement Obligation Disclosure	The following is a summary of our asset retirement obligations: (In Millions) December 31, 2020 2019 Asset retirement obligations 1 $ 342 $ 165 Less: current portion 7 2 Long-term asset retirement obligations $ 335 $ 163 1 Includes $190 million and $22 million related to our active operations as of December 31, 2020 and 2019, respectively. The accrued closure obligation provides for contractual and legal obligations related to our indefinitely idled and closed operations and for the eventual closure of our active operations. We performed a detailed assessment of our asset retirement obligations related to our active operations most recently in 2020 in accordance with our accounting policy, which requires us to perform an in-depth evaluation of the liability every three years in addition to routine annual assessments. In 2020, we employed third-party specialists to assist in the evaluation. Additionally, we have included in our asset retirement obligation $172 million for our integrated steel facilities and other operations acquired in the Acquisitions. The closure date for each of our active mine sites was determined based on the exhaustion date of the remaining mineral reserves and the amortization of the related asset and accretion of the liability is recognized over the estimated mine lives. The closure date and expected timing of the capital requirements to meet our obligations for our indefinitely idled or closed mines is determined based on the unique circumstances of each property. For indefinitely idled or closed mines, the accretion of the liability is recognized over the anticipated timing of remediation. As the majority of our asset retirement obligations at our steelmaking operations have indeterminate settlement dates, asset retirement obligations have been recorded at present values using estimated ranges of the economic lives of the underlying assets. The following is a roll-forward of our asset retirement obligation liability: (In Millions) 2020 2019 Asset retirement obligation as of January 1 $ 165 $ 172 Increase from acquisitions 172 — Accretion expense 14 10 Remediation payments (9) (1) Revision in estimated cash flows — (16) Asset retirement obligation as of December 31 $ 342 $ 165 The revision in estimated cash flows during the year ended December 31, 2019 for $16 million primarily relates to an extension of the life-of-mine plan for Tilden mine based on the economic reserve analysis performed during 2019.

DERIVATIVE INSTRUMENTS

DERIVATIVE INSTRUMENTS	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE INSTRUMENTS	Derivatives Not Designated as Hedging Instruments Customer Supply Agreement A supply agreement with ArcelorMittal USA provided us supplemental revenue or provided refunds to ArcelorMittal USA based on the HRC price at the time the iron ore product was consumed in its blast furnaces. The supplemental pricing is characterized as a freestanding derivative instrument and is required to be accounted for separately once control transfers to the customer. The derivative instrument, which is finalized based on a future price, is adjusted to fair value through Revenues each reporting period based upon current market data and forward-looking estimates provided by management until the pellets are consumed and the amounts are settled. Upon the completion of the AM USA Transaction, the outstanding derivatives were settled as part of acquisition accounting. Included within Revenues related to Topic 815 for the supplemental revenue portion of the supply agreement is derivative revenue of $122 million, $78 million and $426 million for the years ended December 31, 2020, 2019 and 2018, respectively. Refer to NOTE 9 - FAIR VALUE OF FINANCIAL INSTRUMENTS for additional information.

CAPITAL STOCK

CAPITAL STOCK	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
CAPITAL STOCK	Acquisition of AK Steel As more fully described in NOTE 3 - ACQUISITIONS, we acquired AK Steel on March 13, 2020. At the effective time of the AK Steel Merger, each share of AK Steel common stock issued and outstanding prior to the effective time of the AK Steel Merger was converted into, and became exchangeable for, 0.400 Cliffs common shares, par value $0.125 per share. We issued a total of 127 million Cliffs common shares in connection with the AK Steel Merger at a fair value of $618 million. Following the closing of the AK Steel Merger, AK Steel's common stock was de-listed from the NYSE. Acquisition of ArcelorMittal USA As more fully described in NOTE 3 - ACQUISITIONS, we acquired ArcelorMittal USA on December 9, 2020. Pursuant to the terms of the AM USA Transaction Agreement, we issued 78,186,671 common shares and 583,273 shares of a new series of our Serial Preferred Stock, Class B, without par value, designated as the “Series B Participating Redeemable Preferred Stock,” in each case to an indirect, wholly owned subsidiary of ArcelorMittal as part of the consideration paid by us in connection with the closing of the AM USA Transaction. Refer to Preferred Stock below for further information. Preferred Stock We have 3,000,000 shares of Serial Preferred Stock, Class A, without par value and 4,000,000 shares of Serial Preferred Stock, Class B, without par value, authorized; no Class A preferred shares are issued or outstanding. Pursuant to the terms of the AM USA Transaction Agreement, we issued 583,273 shares of a new series of our Serial Preferred Stock, Class B, without par value, designated Series B Participating Redeemable Preferred Stock, without par value, to an indirect, wholly owned subsidiary of ArcelorMittal on December 9, 2020. Series B Participating Redeemable Preferred Stock Terms The Series B Participating Redeemable Preferred Stock is classified for accounting purposes as temporary equity as a result of a change in control provision that could, under remote circumstances, require us to redeem the preferred stock for cash. The Series B Participating Redeemable Preferred Stock ranks senior to our common shares with respect to dividend rights and rights on the distribution of assets upon any voluntary or involuntary liquidation, dissolution or winding up of the affairs of, and certain bankruptcy events involving, Cliffs. Each share of Series B Participating Redeemable Preferred Stock entitles its holder to receive a multiple, initially equal to 100 (subject to certain anti-dilution adjustments, the “Applicable Multiple”), of the aggregate amount per share of all dividends declared on the common shares. In addition, from and after the 24-month anniversary of the issue date of the Series B Participating Redeemable Preferred Stock (the “24-Month Anniversary”), each holder of a share of Series B Participating Redeemable Preferred Stock is entitled to receive cash dividends (the “Additional Dividends”) accruing and compounding on a daily basis at the initial rate of 10% per annum on the sum of (i) the Applicable Multiple then in effect times the volume-weighted average price of the common shares for the 20 consecutive trading days ending on the trading day immediately preceding the 24-Month Anniversary and (ii) the amount of accumulated and unpaid dividends on the Series B Participating Redeemable Preferred Stock to, but not including, the 24-Month Anniversary, if any, which rate will increase by 2% per annum at the end of each six-month period following the 24-Month Anniversary. Additional Dividends will be payable, when, as and if declared by the Board, in quarterly installments. The Series B Participating Redeemable Preferred Stock is redeemable, in whole or in part, at our option at any time and from time to time on and after the date that is 180 days after the issue date at a redemption price per share equal to the Applicable Multiple then in effect times the volume-weighted average price of the common shares for the 20 consecutive trading days ending on the trading day immediately preceding the date fixed for redemption, plus accumulated and unpaid dividends to, but not including, the redemption date. In the event of a change of control of Cliffs, the Series B Participating Redeemable Preferred Stock will be subject to mandatory redemption at a redemption price per share equal to the Applicable Multiple then in effect times the volume-weighted average price of the common shares for the 20 consecutive trading days ending on the trading day immediately preceding the closing date of the transaction constituting such change of control. In addition, pursuant to the terms of the Series B Participating Redeemable Preferred Stock, we are restricted from effecting any merger or consolidation with or into another entity unless the Series B Participating Redeemable Preferred Stock remains outstanding following the merger or consolidation, is exchanged for new preferred stock with substantially identical terms or is to be redeemed in connection with the closing of such merger or consolidation. In addition to the foregoing, the Series B Participating Redeemable Preferred Stock is subject to the express terms of the Serial Preferred Stock, Class B, without par value, as set forth in Cleveland-Cliffs Inc.’s Fourth Amended Articles of Incorporation, as amended, except that holders of Series B Participating Redeemable Preferred Stock, in their capacity as such, do not have the right to vote with the other series of Serial Preferred Stock, Class B, without par value, then outstanding, if any, voting separately as a class, for the election of additional directors of Cleveland-Cliffs Inc. upon certain defaults by the Company in the payment of dividends, as provided in Cleveland-Cliffs Inc.’s Fourth Amended Articles of Incorporation, as amended. Dividends The below table summarizes our recent dividend activity: Declaration Date Record Date Payment Date Dividend Declared per Common Share 1 2/18/2020 4/3/2020 4/15/2020 $ 0.06 12/2/2019 1/3/2020 1/15/2020 0.06 9/3/2019 10/4/2019 10/15/2019 0.10 5/31/2019 7/5/2019 7/15/2019 0.06 2/19/2019 4/5/2019 4/15/2019 0.05 10/18/2018 1/4/2019 1/15/2019 0.05 1 The dividend declared on September 3, 2019 included a special cash dividend of $0.04 per common share. Subsequent to the dividend paid on April 15, 2020, our Board temporarily suspended future dividends as a result of the COVID-19 pandemic in order to preserve cash during this time of economic uncertainty. Share Repurchase Program In November 2018, we announced that our Board of Directors authorized a program to repurchase outstanding common shares in the open market or in privately negotiated transactions, up to a maximum of $200 million, excluding commissions and fees. In April 2019, we announced that our Board of Directors increased the common share repurchase authorization by an additional $100 million, excluding commissions and fees. During 2019, we repurchased 24 million common shares at a cost of $253 million in the aggregate, including commissions and fees. The share repurchase program was effective until December 31, 2019.

ACCUMULATED OTHER COMPREHENSIVE

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Comprehensive Income (Loss) Note [Text Block]	The components of Accumulated other comprehensive loss within Cliffs shareholders’ equity and related tax effects allocated to each are shown below: (In Millions) Pre-tax Tax After-tax As of December 31, 2020: Postretirement benefit liability $ (221) $ 86 $ (135) Foreign currency translation adjustments 3 — 3 Unrealized net loss on derivative financial instruments (1) — (1) $ (219) $ 86 $ (133) As of December 31, 2019: Postretirement benefit liability $ (454) $ 138 $ (316) Unrealized net loss on derivative financial instruments (4) 1 (3) $ (458) $ 139 $ (319) As of December 31, 2018: Postretirement benefit liability $ (408) $ 127 $ (281) Unrealized net loss on derivative financial instruments (4) 1 (3) $ (412) $ 128 $ (284) The following table reflects the changes in Accumulated other comprehensive loss related to Cliffs shareholders’ equity: (In Millions) Postretirement Benefit Liability, Foreign Currency Translation Derivative Financial Instruments, Accumulated Other Comprehensive Loss December 31, 2017 $ (264) $ 225 $ — $ (39) Other comprehensive income (loss) before reclassifications (43) 3 (1) (41) Net loss (gain) reclassified from accumulated other comprehensive loss 26 (228) (2) (204) December 31, 2018 (281) — (3) (284) Other comprehensive loss before reclassifications (57) — (2) (59) Net loss reclassified from accumulated other comprehensive loss 22 — 2 24 December 31, 2019 (316) — (3) (319) Other comprehensive income (loss) before reclassifications 163 3 (6) 160 Net loss reclassified from accumulated other comprehensive loss 18 — 8 26 December 31, 2020 $ (135) $ 3 $ (1) $ (133) The following table reflects the details about Accumulated other comprehensive loss components reclassified from Cliffs shareholders’ equity: (In Millions) Details about Accumulated Other Comprehensive Loss Components Amount of (Gain)/Loss Affected Line Item in the Statement of Consolidated Operations Year Ended December 31, 2020 2019 2018 Amortization of pension and OPEB liability: Prior service costs 1 $ (1) $ (1) $ (1) Other non-operating income Net actuarial loss 1 30 29 27 Other non-operating income Settlements 1 (6) — — Other non-operating income 23 28 26 Total before taxes Income tax expense (5) (6) — Income tax benefit (expense) $ 18 $ 22 $ 26 Net of taxes Changes in foreign currency translation: Gain on foreign currency translation 2 $ — $ — $ (228) Income (loss) from discontinued operations, net of tax Changes in derivative financial instruments: Commodity contracts $ 10 $ 3 $ (2) Cost of goods sold Income tax expense (2) (1) — Income tax benefit (expense) $ 8 $ 2 $ (2) Net of taxes Total reclassifications for the period, net of tax $ 26 $ 24 $ (204) 1 These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost. See NOTE 10 - PENSIONS AND OTHER POSTRETIREMENT BENEFITS for further information. 2 Represents Australian accumulated currency translation adjustments due to the liquidation of our Australian subsidiaries' net assets.

RELATED PARTIES

RELATED PARTIES	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
RELATED PARTIES	We have certain co-owned joint ventures with companies from the steel and mining industries, including integrated steel companies, their subsidiaries and other downstream users of steel and iron ore products. Hibbing is a co-owned joint venture with U.S. Steel, in which, as of December 31, 2020, we own 85.3% and U.S. Steel owns 14.7%. As a result of the AM USA Transaction, we acquired an additional 62.3% ownership stake in the Hibbing mine and became the majority owner and mine manager. Prior to the AM USA Transaction, ArcelorMittal was a related party due to its ownership interest in Hibbing. As such, certain long-term contracts with ArcelorMittal resulted in Revenues from related parties, and are included within the below. Revenues from related parties were as follows: (In Millions) Year Ended December 31, 2020 2019 2018 Revenue from related parties $ 893 $ 1,015 $ 1,344 Revenues 1 $ 5,354 $ 1,990 $ 2,332 Related party revenues as a percent of Revenues 1 16.7 % 51.0 % 57.6 % Purchases from related parties $ 16 $ — $ — 1 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020. The following table presents the classification of related party assets and liabilities in the Statements of Consolidated Financial Position: (In Millions) December 31, Balance Sheet Location of Assets (Liabilities) 2020 2019 Accounts receivable, net $ 2 $ 31 Other current assets — 45 Accounts payable (6) — Other current liabilities — (2) Other current assets Our supply agreement with ArcelorMittal USA contained provisions that provided us supplemental revenue or provided refunds to ArcelorMittal USA based on the HRC price at the time the iron ore product was consumed in its blast furnaces. The supplemental pricing was categorized as a freestanding derivative. Upon the completion of the AM USA Transaction, the outstanding derivative was settled as part of acquisition accounting.

VARIABLE INTEREST ENTITIES

VARIABLE INTEREST ENTITIES	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable Interest Entity Disclosure	SunCoke Middletown We purchase all the coke and electrical power generated from SunCoke Middletown’s plant under long-term supply agreements and have committed to purchase all the expected production from the facility through 2032. We consolidate SunCoke Middletown as a VIE because we are the primary beneficiary despite having no ownership interest in SunCoke Middletown. SunCoke Middletown had income before income taxes of $41 million for the year ended December 31, 2020, which was included in our consolidated income before income taxes. The assets of the consolidated VIE can only be used to settle the obligations of the consolidated VIE and not obligations of the Company. The creditors of SunCoke Middletown do not have recourse to the assets or general credit of the Company to satisfy liabilities of the VIE. The consolidated balance sheet as of December 31, 2020 includes the following amounts for SunCoke Middletown: (In Millions) December 31, Cash and cash equivalents $ 5 Inventories 21 Property, plant and equipment, net 308 Accounts payable (15) Other assets (liabilities), net (10) Noncontrolling interest (309)

EARNINGS PER SHARE

EARNINGS PER SHARE	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
EARNINGS PER SHARE	The following table summarizes the computation of basic and diluted EPS: (In Millions, Except Per Share Amounts) Year Ended December 31, 2020 2019 2018 Income (loss) from continuing operations $ (82) $ 295 $ 1,040 Income from continuing operations attributable to noncontrolling interest (41) — — Net income (loss) from continuing operations attributable to Cliffs shareholders (123) 295 1,040 Income (loss) from discontinued operations, net of tax 1 (2) 88 Net income (loss) attributable to Cliffs shareholders $ (122) $ 293 $ 1,128 Weighted average number of shares: Basic 379 277 297 Convertible senior notes — 4 3 Employee stock plans — 3 4 Diluted 379 284 304 Earnings (loss) per common share attributable to Continuing operations $ (0.32) $ 1.07 $ 3.50 Discontinued operations — (0.01) 0.30 $ (0.32) $ 1.06 $ 3.80 Earnings (loss) per common share attributable to Continuing operations $ (0.32) $ 1.04 $ 3.42 Discontinued operations — (0.01) 0.29 $ (0.32) $ 1.03 $ 3.71 The following table summarizes the shares that have been excluded from the diluted earnings per share calculation for the year ended December 31, 2020, as they were anti-dilutive: (In Millions) 2020 Redeemable preferred shares 4 Convertible senior notes 2 Shares related to employee stock plans 1

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	Purchase Commitments We purchase portions of the principal raw materials required for our steel manufacturing operations under annual and multi-year agreements, some of which have minimum quantity requirements. We also use large volumes of natural gas, electricity and industrial gases in our steel manufacturing operations. We negotiate most of our purchases of chrome, industrial gases and a portion of our electricity under multi-year agreements. Our purchases of coke are made under annual or multi-year agreements with periodic price adjustments. We typically purchase coal under annual fixed-price agreements. We also purchase certain transportation services under multi-year contracts with minimum quantity requirements. Contingencies We are currently the subject of, or party to, various claims and legal proceedings incidental to our current and historical operations. These claims and legal proceedings are subject to inherent uncertainties and unfavorable rulings could occur. An unfavorable ruling could include monetary damages, additional funding requirements or an injunction. If an unfavorable ruling were to occur, there exists the possibility of a material adverse effect on the financial position and results of operations for the period in which the ruling occurs or future periods. However, based on currently available information we do not believe that any pending claims or legal proceedings will result in a material adverse effect in relation to our consolidated financial statements. Environmental Contingencies Although we believe our operating practices have been consistent with prevailing industry standards, hazardous materials may have been released at operating sites or third-party sites in the past, including operating sites that we no longer own. If we reasonably can, we estimate potential remediation expenditures for those sites where future remediation efforts are probable based on identified conditions, regulatory requirements, or contractual obligations arising from the sale of a business or facility. For sites involving government required investigations, we typically make an estimate of potential remediation expenditures only after the investigation is complete and when we better understand the nature and scope of the remediation. In general, the material factors in these estimates include the costs associated with investigations, delineations, risk assessments, remedial work, governmental response and oversight, site monitoring, and preparation of reports to the appropriate environmental agencies. The following is a summary of our environmental obligations: (In Millions) December 31, December 31, Environmental obligations $ 135 $ 2 Less current portion 18 — Long-term environmental obligations $ 117 $ 2 We cannot predict the ultimate costs for each site with certainty because of the evolving nature of the investigation and remediation process. Rather, to estimate the probable costs, we must make certain assumptions. The most significant of these assumptions is for the nature and scope of the work that will be necessary to investigate and remediate a particular site and the cost of that work. Other significant assumptions include the cleanup technology that will be used, whether and to what extent any other parties will participate in paying the investigation and remediation costs, reimbursement of past response costs and future oversight costs by governmental agencies, and the reaction of the governing environmental agencies to the proposed work plans. Costs for future investigation and remediation are not discounted to their present value, unless the amount and timing of the cash disbursements are readily known. To the extent that we have been able to reasonably estimate future liabilities, we do not believe that there is a reasonable possibility that we will incur a loss or losses that exceed the amounts we accrued for the environmental matters discussed below that would, either individually or in the aggregate, have a material adverse effect on our consolidated financial condition, results of operations or cash flows. However, since we recognize amounts in the consolidated financial statements in accordance with GAAP that exclude potential losses that are not probable or that may not be currently estimable, the ultimate costs of these environmental matters may be higher than the liabilities we currently have recorded in our consolidated financial statements. Pursuant to RCRA, which governs the treatment, handling and disposal of hazardous waste, the EPA and authorized state environmental agencies may conduct inspections of RCRA-regulated facilities to identify areas where there have been releases of hazardous waste or hazardous constituents into the environment and may order the facilities to take corrective action to remediate such releases. Likewise, the EPA or the states may require closure or post-closure care of residual, industrial and hazardous waste management units, including, but not limited to, landfills and deep injection wells. Environmental regulators have the authority to inspect all of our facilities. While we cannot predict the future actions of these regulators, it is possible that they may identify conditions in future inspections of these facilities that they believe require corrective action. Pursuant to CERCLA, the EPA and state environmental authorities have conducted site investigations at some of our facilities and other third-party facilities, portions of which previously may have been used for disposal of materials that are currently regulated. The results of these investigations are still pending, and we could be directed to spend funds for remedial activities at the former disposal areas. Because of the uncertain status of these investigations, however, we cannot reasonably predict whether or when such spending might be required or its magnitude. On April 29, 2002, AK Steel entered a mutually agreed-upon administrative order with the consent of the EPA pursuant to Section 122 of CERCLA to perform a RI/FS of the Hamilton plant site located in New Miami, Ohio. The plant ceased operations in 1990 and all of its former structures have been demolished. AK Steel submitted the investigation portion of the RI/FS and completed supplemental studies. Until the RI/FS is complete, we cannot reasonably estimate the additional costs, if any, we may incur for potentially required remediation of the site or when we may incur them. EPA Administrative Order In Re: Ashland Coke On September 26, 2012, the EPA issued an order under Section 3013 of RCRA requiring a plan to be developed for investigation of four areas at the Ashland Works coke plant. The Ashland Works coke plant ceased operations in 2011 and all of its former structures have been demolished and removed. In 1981, AK Steel acquired the plant from Honeywell International Corporation (as successor to Allied Corporation), who had managed the coking operations there for approximately 60 years. In connection with the sale of the coke plant, Honeywell agreed to indemnify AK Steel against certain claims and obligations that could arise from the investigation, and we intend to pursue such indemnification from Honeywell, if necessary. We cannot reasonably estimate how long it will take to complete the site investigation. On March 10, 2016, the EPA invited AK Steel to participate in settlement discussions regarding an enforcement action. Settlement discussions between the parties are ongoing, though whether the parties will reach agreement and any such agreement’s terms are uncertain. Until the site investigation is complete, we cannot reasonably estimate the costs, if any, we may incur for potential additional required remediation of the site or when we may incur them. Burns Harbor Water Issues In August 2019, ArcelorMittal Burns Harbor LLC (n/k/a Cleveland-Cliffs Burns Harbor LLC) suffered a loss of the blast furnace cooling water recycle system, which led to the discharge of cyanide and ammonia in excess of the Burns Harbor plant's NPDES permit limits. Since that time, the facility has taken numerous steps to prevent recurrence and maintain compliance with its NPDES permit. Since the August 2019 event, we have been engaged in settlement discussions with the U.S. Department of Justice, the EPA and the State of Indiana to resolve any alleged violations of environmental laws or regulations. Also, ArcelorMittal Burns Harbor LLC was served with a subpoena on December 5, 2019, from the United States District Court for the Northern District of Indiana relating to the August 2019 event and has responded to the subpoena requests. In addition, the plaintiffs in Environmental Law & Policy Center et al. v. ArcelorMittal Burns Harbor LLC et al. (U.S. District Court, N.D. Indiana Case No. 19-cv-473) , which was filed on December 20, 2019, have alleged violations resulting from the August 2019 event and other Clean Water Act claims. Although we cannot accurately estimate the amount of civil penalty, the cost of any injunctive relief requirements, or the costs to resolve third-party claims, including potential natural resource damages claims, they are likely to exceed the reporting threshold in total. In addition to the foregoing matters, we are or may be involved in proceedings with various regulatory authorities that may require us to pay fines, comply with more rigorous standards or other requirements or incur capital and operating expenses for environmental compliance. We believe that the ultimate disposition of any such proceedings will not have, individually or in the aggregate, a material adverse effect on our consolidated financial condition, results of operations or cash flows. Tax Matters The calculation of our tax liabilities involves dealing with uncertainties in the application of complex tax regulations. We recognize liabilities for anticipated tax audit issues based on our estimate of whether, and the extent to which, additional taxes will be due. If we ultimately determine that payment of these amounts is unnecessary, we reverse the liability and recognize a tax benefit during the period in which we determine that the liability is no longer necessary. We also recognize tax benefits to the extent that it is more likely than not that our positions will be sustained when challenged by the taxing authorities. To the extent we prevail in matters for which liabilities have been established, or are required to pay amounts in excess of our liabilities, our effective tax rate in a given period could be materially affected. An unfavorable tax settlement would require use of our cash and result in an increase in our effective tax rate in the year of resolution. A favorable tax settlement would be recognized as a reduction in our effective tax rate in the year of resolution. Refer to NOTE 12 - INCOME TAXES for further information. Other Contingencies In addition to the matters discussed above, there are various pending and potential claims against us and our subsidiaries involving product liability, commercial, employee benefits and other matters arising in the ordinary course of business. Because of the considerable uncertainties that exist for any claim, it is difficult to reliably or accurately estimate what the amount of a loss would be if a claimant prevails. If material assumptions or factual understandings we rely on to evaluate exposure for these contingencies prove to be inaccurate or otherwise change, we may be required to record a liability for an adverse outcome. If, however, we have reasonably evaluated potential future liabilities for all of these contingencies, including those described more specifically above, it is our opinion, unless we otherwise noted, that the ultimate liability from these contingencies, individually or in the aggregate, should not have a material adverse effect on our consolidated financial position, results of operations or cash flows.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	On February 11, 2021, we sold 20 million common shares, and the indirect, wholly owned subsidiary of ArcelorMittal to which approximately 78 million common shares were issued as part of the consideration paid by us in connection with the closing of the AM USA Transaction sold 40 million common shares, in each case at a price per share to the underwriter of $16.12, in an underwritten public offering. We also granted the underwriter an option to purchase up to an additional 9 million common shares from us at a price per share to the underwriter of $16.12. The underwriter has until March 10, 2021 to exercise such option, which it may do in full, in part or not at all. We did not receive any proceeds from the sale of the common shares by the selling shareholder in the offering. We intend to use the net proceeds to us from the offering, plus cash on hand, to redeem up to approximately $334 million aggregate principal amount of our outstanding 9.875% 2025 Senior Secured Notes. We intend to use any remaining net proceeds to us following such redemption to reduce borrowings under our ABL Facility. On February 9, 2021, following pricing of the underwritten public offering, we issued a conditional notice of partial redemption to holders of the 9.875% 2025 Senior Secured Notes to redeem $322 million aggregate principal amount of the outstanding 9.875% 2025 Senior Secured Notes on March 11, 2021, at a redemption price equal to 109.875% of the principal amount to be redeemed, plus accrued and unpaid interest, if any, to, but excluding, the redemption date. The conditional notice of partial redemption was subject to the condition precedent that we close the underwritten public offering of our common shares. As a result, following the closing of the underwritten public offering, the conditional notice of partial redemption became irrevocable. On February 17, 2021, we issued $500 million aggregate principal amount of 4.625% 2029 Senior Notes and $500 million aggregate principal amount of 4.875% 2031 Senior Notes in an offering that was exempt from the registration requirements of the Securities Act. We intend to use the net proceeds from the notes offering to redeem all of the outstanding 4.875% 2024 Senior Secured Notes and 6.375% 2025 Senior Notes issued by Cleveland-Cliffs Inc. and all of the outstanding 7.625% 2021 AK Senior Notes, 7.50% 2023 AK Senior Notes and 6.375% 2025 AK Senior Notes issued by AK Steel Corporation (n/k/a Cleveland-Cliffs Steel Corporation), and pay fees and expenses in connection with such redemptions, and reduce borrowings under our ABL Facility. On February 10, 2021, following pricing of the notes offering, we issued notices of redemption to the holders of the 4.875% 2024 Senior Secured Notes, 6.375% 2025 Senior Notes, 7.625% 2021 AK Senior Notes, 7.50% 2023 AK Senior Notes and 6.375% 2025 AK Senior Notes to redeem all of such notes outstanding on March 12, 2021, at the redemption prices listed below, plus accrued and unpaid interest, if any, to, but excluding, the redemption date. The notices of redemption with respect to the 4.875% 2024 Senior Secured Notes and the 6.375% 2025 Senior Notes were subject to the condition precedent that we close the notes offering. As a result, following the closing of the notes offering, the notices of redemption with respect to the 4.875% 2024 Senior Secured Notes and the 6.375% 2025 Senior Notes became irrevocable. The notices of redemption with respect to the 7.625% 2021 AK Senior Notes, 7.50% 2023 AK Senior Notes and 6.375% 2025 AK Senior Notes were not subject to any conditions and were irrevocable when issued. Debt Instrument Redemption Price 1 4.875% 2024 Senior Secured Notes 102.438 % 6.375% 2025 Senior Notes 103.188 7.625% 2021 AK Senior Notes 100.000 7.50% 2023 AK Senior Notes 101.875 6.375% 2025 AK Senior Notes 103.188 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date.

BASIS OF PRESENTATION AND SIG_2

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Consolidation	Basis of Consolidation The condensed consolidated financial statements consolidate our accounts and the accounts of our wholly owned subsidiaries, all subsidiaries in which we have a controlling interest and VIEs for which we are the primary beneficiary. All intercompany transactions and balances are eliminated upon consolidation.
Equity Method Investments	Investments in Affiliates We have investments in several businesses accounted for using the equity method of accounting. These investments are included within our Steelmaking segment. We review an investment for impairment when circumstances indicate that a loss in value below its carrying amount is other than temporary. Investees and equity ownership percentages are presented below: Investee Equity Ownership Percentage Combined Metals of Chicago, LLC 40.0% Spartan Steel Coating, LLC 48.0% As of December 31, 2019, our 23% ownership in Hibbing was recorded as an equity method investment. As a result of the acquisition of ArcelorMittal USA, we acquired an additional 62.3% ownership interest in Hibbing. As of December 31, 2020, our ownership in the Hibbing joint venture was 85.3% and was fully consolidated within our operating results with a noncontrolling interest. We recorded a basis difference for Spartan Steel of $33 million as part of the AK Steel Merger. The basis difference relates to the excess of the fair value over the investee's carrying amount of property, plant and equipment and will be amortized over the remaining useful lives of the underlying assets. As of December 31, 2020, our investment in affiliates of $105 million was classified in Other non-current assets . As of December 31, 2019, our investment in affiliates of $18 million was classified in Other non-current liabilities.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Our mineral reserves; future realizable cash flow; environmental, reclamation and closure obligations; valuation of business combinations, long-lived assets, inventory, tax assets and post-employment, post-retirement and other employee benefit liabilities; reserves for contingencies and litigation require the use of various management estimates and assumptions. Actual results could differ from estimates. Management reviews its estimates on an ongoing basis. Changes in facts and circumstances may alter such estimates and affect the results of operations and financial position in future periods.
Business Combinations Policy	Business Combinations Assets acquired and liabilities assumed in a business combination are recognized and measured based on their estimated fair values at the acquisition date, while the acquisition-related costs are expensed as incurred. Any excess of the purchase consideration when compared to the fair value of the net tangible and intangible assets acquired, if any, is recorded as goodwill. We engaged independent valuation specialists to assist with the determination of the fair value of assets acquired, liabilities assumed, noncontrolling interest, and goodwill, for the Acquisitions. If the initial accounting for the business combination is incomplete by the end of the reporting period in which the acquisition occurs, an estimate will be recorded. Subsequent to the acquisition date, and not later than one year from the acquisition date, we will record any material adjustments to the initial estimate based on new information obtained that would have existed as of the date of the acquisition. Any adjustment that arises from information obtained that did not exist as of the date of the acquisition will be recorded in the period the adjustment arises.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include cash on hand and on deposit as well as all short-term securities held for the primary purpose of general liquidity. We consider investments in highly liquid debt instruments with an original maturity of three months or less from the date of acquisition and longer maturities when funds can be withdrawn in three months or less without a significant penalty to be cash equivalents. We routinely monitor and evaluate counterparty credit risk related to the financial institutions in which our short-term investment securities are held.
Receivables, Trade and Other Accounts Receivable, Allowance for Doubtful Accounts, Policy	Trade Accounts Receivable and Allowance for Credit LossTrade accounts receivable are recorded at the point control transfers and represent the amount of consideration we expect to receive in exchange for transferred goods and do not bear interest. We establish provisions for expected lifetime losses on accounts receivable at the time a receivable is recorded based on historical experience, customer credit quality and forecasted economic conditions. We regularly review our accounts receivable balances and the allowance for credit loss and establish or adjust the allowance as necessary using the specific identification method in accordance with CECL. We evaluate the aggregation and risk characteristics of receivable pools and develop loss rates that reflect historical collections, current forecasts of future economic conditions over the time horizon we are exposed to credit risk, and payment terms or conditions that may materially affect future forecasts.
Inventories	Inventories Inventories are generally stated at the lower of cost or net realizable value using average cost, excluding depreciation and amortization. Certain iron ore inventories are stated at the lower of cost or market using the LIFO method. Refer to NOTE 2 - SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION for further information.
Derivative Financial Instruments and Hedging Activities	Derivative Financial Instruments and Hedging Activities We are exposed to certain risks related to the ongoing operations of our business, including those caused by changes in commodity prices and energy rates. We have established policies and procedures, including the use of certain derivative instruments, to manage such risks, if deemed necessary. Derivative financial instruments are recognized as either assets or liabilities in the Statements of Consolidated Financial Position and measured at fair value. On the date a qualifying hedging instrument is executed, we designate the hedging instrument as a hedge of the variability of cash flows to be received or paid related to a forecasted transaction (cash flow hedge). We formally document all relationships between hedging instruments and hedged items, as well as our risk-management objective and strategy for undertaking various hedge transactions. This process includes linking all derivatives that are designated as cash flow hedges to specific firm commitments or forecasted transactions. We also formally assess, both at the hedge's inception and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows of the related hedged items. When it is determined that a derivative is not highly effective as a hedge, we discontinue hedge accounting prospectively and record all future changes in fair value in the period of the instrument's earnings or losses. For derivative instruments that have been designated as cash flow hedges, the changes in fair value are recorded in Accumulated other comprehensive loss . Amounts recorded in Accumulated other comprehensive loss are reclassified to earnings or losses in the period the underlying hedged transaction affects earnings or when the underlying hedged transaction is no longer reasonably possible of occurring. For derivative instruments that have not been designated as cash flow hedges, such as provisional pricing arrangements, changes in fair value are recorded in the period of the instrument's earnings or losses. Refer to Revenue Recognition below for discussion of derivatives recorded as a result of pricing terms in our sales contracts. Additionally, refer to NOTE 15 - DERIVATIVE INSTRUMENTS for further information.
Property, Plant and Equipment	Property, Plant and Equipment Our properties are stated at the lower of cost less accumulated depreciation. Depreciation of plant and equipment is computed principally by the straight-line method based on estimated useful lives. Depreciation continues to be recognized when operations are idled temporarily. Depreciation and depletion are recorded over the following estimated useful lives: Asset Class Basis Life Land, land improvements and mineral rights Land and mineral rights Units of production Life of mine Land improvements Straight line 20 to 45 years Buildings Straight line 20 to 45 years Equipment Straight line/Double declining balance 3 to 20 years Refer to NOTE 6 - PROPERTY, PLANT AND EQUIPMENT for further information.
Goodwill and Other Intangible Assets and Liabilities	Goodwill Goodwill represents the excess purchase price paid over the fair value of the net assets during an acquisition. Goodwill is not amortized, but is assessed for impairment on an annual basis on October 1 (or more frequently if necessary). Other Intangible Assets and Liabilities Intangible assets and liabilities are subject to periodic amortization on a straight-line basis over their estimated useful lives as follows: Type Basis Useful Life Intangible assets: Customer relationships Straight line 18 years Developed technology Straight line 17 years Trade names and trademarks Straight line 10 years Mining permits Straight line Life of mine Intangible liabilities: Above-market supply contracts Straight line Contract term Refer to NOTE 7 - GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES for further information.
Leases	Leases We determine if an arrangement contains a lease at inception. We recognize right-of-use assets and lease liabilities associated with leases based on the present value of the future minimum lease payments over the lease term at the commencement date. Lease terms reflect options to extend or terminate the lease when it is reasonably certain that the option will be exercised. For short-term leases (leases with an initial lease term of 12 months or less), right-of-use assets and lease liabilities are not recognized in the consolidated balance sheet, and lease expense is recognized on a straight-line basis over the lease term. Refer to NOTE 13 - LEASE OBLIGATIONS for further information.
Asset Impairment	Asset Impairment We monitor conditions that may affect the carrying value of our long-lived tangible and intangible assets when events and circumstances indicate that the carrying value of the asset groups may not be recoverable. In order to determine if assets have been impaired, assets are grouped and tested at the lowest level for which identifiable, independent cash flows are available ("asset group"). The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Fair value can be determined using a market approach, income approach or cost approach. For the years ended December 31, 2020, 2019 and 2018, no impairment indicators were present that would indicate the carrying value of any of our asset groups may not be recoverable; as a result, no impairment assessments were required.
Fair Value Measurements	Fair Value Measurements ASC Topic 820, Fair Value Measurements and Disclosures , establishes a three-level valuation hierarchy for classification of fair value measurements. The valuation hierarchy is based upon the transparency of inputs to the valuation of an asset or liability as of the measurement date. Inputs refer broadly to the assumptions that market participants would use in pricing an asset or liability. Inputs may be observable or unobservable. Observable inputs are inputs that reflect the assumptions market participants would use in pricing the asset or liability developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our own views about the assumptions market participants would use in pricing the asset or liability developed based on the best information available in the circumstances. The three-tier hierarchy of inputs is summarized below: • Level 1 — Valuation is based upon quoted prices (unadjusted) for identical assets or liabilities in active markets. • Level 2 — Valuation is based upon quoted prices for similar assets and liabilities in active markets, or other inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument. • Level 3 — Valuation is based upon other unobservable inputs that are significant to the fair value measurement. The classification of assets and liabilities within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement in its entirety. Refer to NOTE 9 - FAIR VALUE OF FINANCIAL INSTRUMENTS and NOTE 10 - PENSIONS AND OTHER POSTRETIREMENT BENEFITS for further information.
Pension and Other Postretirement Plans	Pensions and Other Postretirement Benefits We offer defined benefit pension plans, defined contribution pension plans and OPEB plans, primarily consisting of retiree healthcare benefits as part of our total compensation and benefits programs. We recognize the funded or unfunded status of our pension and OPEB obligations on our December 31, 2020 and 2019 Statements of Consolidated Financial Position based on the difference between the market value of plan assets and the actuarial present value of our retirement obligations on that date, on a plan-by-plan basis. If the plan assets exceed the pension and OPEB obligations, the amount of the surplus is recorded as an asset; if the pension and OPEB obligations exceed the plan assets, the amount of the underfunded obligations is recorded as a liability. Year-end balance sheet adjustments to pension and OPEB assets and obligations are recorded as Accumulated other comprehensive loss in the Statements of Consolidated Financial Position. The actuarial estimates of the PBO and APBO incorporate various assumptions including the discount rates, the rates of increases in compensation, healthcare cost trend rates, mortality, retirement timing and employee turnover. The discount rate is determined based on the prevailing year-end rates for high-grade corporate bonds with a duration matching the expected cash flow timing of the benefit payments from the various plans. The remaining assumptions are based on our estimates of future events by incorporating historical trends and future expectations. The amount of net periodic cost that is recorded in the Statements of Consolidated Operations consists of several components including service cost, interest cost, expected return on plan assets, and amortization of previously unrecognized amounts. Service cost represents the value of the benefits earned in the current year by the participants. Interest cost represents the cost associated with the passage of time. Certain items, such as plan amendments, gains and/or losses resulting from differences between actual and assumed results for demographic and economic factors affecting the obligations and assets of the plans, and changes in other assumptions are subject to deferred recognition for income and expense purposes. The expected return on plan assets is determined utilizing the weighted average of expected returns for plan asset investments in various asset categories based on historical performance, adjusted for current trends. Service costs are classified within Cost of goods sold, Selling, general and administrative expenses and Miscellaneous – net while the interest cost, expected return on assets, amortization of prior service costs/credits, net actuarial gain/loss, and other costs are classified within Other non-operating income. Refer to NOTE 10 - PENSIONS AND OTHER POSTRETIREMENT BENEFITS for further information.
Labor Agreements	Labor Agreements At December 31, 2020, we employed approximately 25,000 people, of which approximately 18,500 were represented by labor unions under various agreements. We have agreements that will expire at five locations in 2021 and sixteen locations in 2022. Workers at some of our North American facilities are covered by agreements with the USW or other unions that have various expiration dates.
Asset Retirement Obligations	Asset Retirement Obligations Asset retirement obligations are recognized when incurred and recorded as liabilities at fair value. The fair value of the liability is determined as the discounted value of the expected future cash flows. The asset retirement obligation is accreted over time through periodic charges to earnings. In addition, the asset retirement cost is capitalized and amortized over the life of the related asset. Reclamation costs are adjusted periodically to reflect changes in the estimated present value resulting from the passage of time and revisions to the estimates of either the timing or amount of the reclamation costs. We review, on an annual basis, unless otherwise deemed necessary, the asset retirement obligation for each applicable operation in accordance with the provisions of ASC Topic 410, Asset Retirement and Environmental Obligations . We perform an in-depth evaluation of the liability every three years in addition to our routine annual assessments. Future reclamation costs for inactive operations are accrued based on management’s best estimate at the end of each period of the costs expected to be incurred at a site. Such cost estimates include, where applicable, ongoing maintenance and monitoring costs. Changes in estimates at inactive operations are reflected in earnings in the period an estimate is revised. Refer to NOTE 14 - ASSET RETIREMENT OBLIGATIONS for further information.
Environmental Remediation Costs	Environmental Remediation Costs We have a formal policy for environmental protection and restoration. Certain of our operating activities are subject to various laws and regulations governing protection of the environment. We conduct our operations to protect the public health and environment and believe our operations are in compliance with applicable laws and regulations in all material respects. Our environmental liabilities, including obligations for known environmental remediation exposures, have been recognized based on the estimated cost of investigation and remediation at each site. If the cost can only be estimated as a range of possible amounts with no point in the range being more likely, the minimum of the range is accrued. Future expenditures are discounted unless the amount and timing of the cash disbursements cannot be reasonably estimated. It is possible that additional environmental obligations could be incurred, the extent of which cannot be assessed. Potential insurance recoveries have not been reflected in the determination of the liabilities. Refer to NOTE 21 - COMMITMENTS AND CONTINGENCIES for further information.
Revenue Recognition	Revenue Recognition Sales are recognized when our performance obligations are satisfied. Generally, our performance obligations are satisfied, control of our products is transferred and revenue is recognized at a single point in time, when title transfers to our customer for product shipped according to shipping terms. Shipping and other transportation costs charged to customers are treated as fulfillment activities and are recorded in both revenue and cost of sales at the time control is transferred to the customer. Refer to NOTE 4 - REVENUES for further information.
Repairs And Maintenance	Repairs and Maintenance Repairs, maintenance and replacement of components are expensed as incurred. The cost of major equipment overhauls is capitalized and depreciated over the estimated useful life, which is the period until the next scheduled overhauls. All other planned and unplanned repairs and maintenance costs are expensed when incurred.
Share-based Compensation	Share-Based Compensation The fair value of each performance share grant is estimated on the date of grant using a Monte Carlo simulation to forecast relative TSR performance. A correlation matrix of historical and projected stock prices was developed for both the Company and its predetermined peer group of mining and metals companies. The fair value assumes that the performance objective will be achieved. The expected term of the grant represents the time from the grant date to the end of the service period. We estimate the volatility of our common shares and that of the peer group of mining and metals companies using daily price intervals for all companies. The risk-free interest rate is the rate at the grant date on zero-coupon government bonds, with a term commensurate with the remaining performance period. The fair value of the restricted stock units is determined based on the closing price of our common shares on the grant date. Upon vesting of share-based compensation awards, we issue shares from treasury shares before issuing new shares. Forfeitures are recognized when they occur. The fair value of stock options is estimated on the date of grant using a Black-Scholes model using the grant date price of our common shares and option exercise price, and assumptions regarding the option’s expected term, the volatility of our common shares, the risk-free interest rate, and the dividend yield over the option’s expected term. Refer to NOTE 11 - STOCK COMPENSATION PLANS for additional information.
Income Taxes	Income Taxes Income taxes are based on income for financial reporting purposes, calculated using tax rates by jurisdiction, and reflect a current tax liability or asset for the estimated taxes payable or recoverable on the current year tax return and expected annual changes in deferred taxes. Any interest or penalties on income tax are recognized as a component of Income tax benefit (expense) . We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized within Net income (loss) in the period that includes the enactment date. We record net deferred tax assets to the extent we believe these assets will more likely than not be realized. In making such determination, we consider all available positive and negative evidence, including scheduled reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial results of operations. Accounting for uncertainty in income taxes recognized in the financial statements requires that a tax benefit from an uncertain tax position be recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on technical merits. See NOTE 12 - INCOME TAXES for further information.
Discontinued Operations	Discontinued Operations During 2018, we committed to a course of action leading to the permanent closure of the Asia Pacific Iron Ore mining operations and sold all of the assets of our Asia Pacific Iron Ore business through a series of sales to third parties. As a result of our exit, management determined that our Asia Pacific Iron Ore mining operations met the criteria to be classified as held for sale and a discontinued operation under ASC Topic 205, Presentation of Financial Statements . As such, all current and historical Asia Pacific Iron Ore operating results are classified within discontinued operations.
Foreign Currency	Foreign Currency Our financial statements are prepared with the U.S. dollar as the reporting currency and the functional currency of all subsidiaries is the U.S. dollar, except for our European Operations for which the functional currency is the Euro. In August 2018, management determined that there were significant changes in economic factors related to our Australian subsidiaries. The change in economic factors was a result of the sale and conveyance of substantially all assets and liabilities of our Australian subsidiaries to third parties, representing a significant change in operations. As such, the functional currency for the Australian subsidiaries changed from the Australian dollar to the U.S. dollar and all remaining Australian denominated monetary balances will be remeasured prospectively through the Statements of Consolidated Operations. As a result of the liquidation of the Australian subsidiaries' assets, the historical impact of foreign currency translation recorded in Accumulated other comprehensive loss in the Statements of Consolidated Financial Position of $228 million was reclassified and recognized as a gain in Income (loss) from discontinued operations, net of tax in the Statements of Consolidated Operations for the year ended December 31, 2018.
Earnings Per Share	Earnings Per Share We present both basic and diluted EPS amounts for continuing operations and discontinued operations. Total basic EPS amounts are calculated by dividing Net income (loss) attributable to Cliffs shareholders , less the earnings allocated to our Series B Participating Redeemable Preferred Stock, by the weighted average number of common shares outstanding during the period presented. Total diluted EPS amounts are calculated by dividing Net income (loss) attributable to Cliffs shareholders by the weighted average number of common shares, common share equivalents under stock plans using the treasury-stock method, common share equivalents of the Series B Participating Redeemable Preferred Stock using the if-converted method and the calculated common share equivalents in excess of the conversion rate related to our 1.50% 2025 Convertible Senior Notes using the treasury-stock method. Common share equivalents are excluded from EPS computations in the periods in which they have an anti-dilutive effect. See NOTE 8 - DEBT AND CREDIT FACILITIES and NOTE 20 - EARNINGS PER SHARE for further information.
Variable Interest Entities	Variable Interest Entities We assess whether we have a variable interest in legal entities in which we have a financial relationship and, if so, whether or not those entities are VIEs. A VIE is an entity with insufficient equity at risk for the entity to finance its activities without additional subordinated financial support or in which equity investors lack the characteristics of a controlling financial interest. If an entity is determined to be a VIE, we evaluate whether we are the primary beneficiary. The primary beneficiary analysis is a qualitative analysis based on power and economics. We conclude that we are the primary beneficiary and consolidate the VIE if we have both (i) the power to direct the activities of the VIE that most significantly influence the VIE's economic performance and (ii) the obligation to absorb losses of, or the right to receive benefits from, the VIE that could potentially be significant to the VIE. Refer to NOTE 19 - VARIABLE INTEREST ENTITIES for additional information.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Issued and Adopted On March 2, 2020, the SEC issued a final rule that amended the disclosure requirements related to certain registered securities under SEC Regulation S-X, Rule 3-10, which required separate financial statements for subsidiary issuers and guarantors of registered debt securities unless certain exceptions are met. The final rule replaces the previous requirement under Rule 3-10 to provide condensed consolidating financial information in the registrant’s financial statements with a requirement to provide alternative financial disclosures (which include summarized financial information of the parent and any issuers and guarantors, as well as other qualitative disclosures) in either the registrant’s Management's Discussion and Analysis of Financial Condition and Results of Operations or its financial statements, in addition to other simplifications. The final rule is effective for filings on or after January 4, 2021, and early adoption is permitted. We elected to early adopt this disclosure update for the period ended March 31, 2020. As a result, we have excluded the footnote disclosures required under the previous Rule 3-10, and applied the final rule by including the summarized financial information and qualitative disclosures in Part II - Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations of this Annual Report on Form 10-K and Exhibit 22, filed herewith. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) . The new standard requires lessees to recognize a right-of-use asset and a lease liability on the balance sheet for all leases except for short-term leases. For lessees, leases are classified as either operating or finance leases. We adopted this standard on its effective date of January 1, 2019 using the optional alternative approach, which requires application of the new guidance at the beginning of the standard's effective date. Adoption of the updated standard did not have a material effect on our consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326) , which introduces a new accounting model, CECL. CECL requires earlier recognition of credit losses, while also providing additional transparency about credit risk. CECL utilizes a lifetime expected credit loss measurement objective for the recognition of credit losses at the time the financial asset is originated or acquired. The expected credit losses are adjusted each period for changes in expected lifetime credit losses. We elected to early adopt this standard on December 31, 2019. Upon adoption, the updated standard did not have a material effect on our consolidated financial statements. Issued and Not Effective In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40). This update requires certain convertible instruments to be accounted for as a single liability measured at its amortized cost. Additionally, the update requires the use of the "if-converted" method, removing the treasury stock method, when calculating diluted shares. The two methods of adoption are the full and modified retrospective approaches. We expect to utilize the modified retrospective approach. Using this approach, the guidance shall be applied to transactions outstanding as of the beginning of the fiscal year in which the amendment is adopted. The final rule is effective for fiscal years beginning after December 15, 2021. Early adoption is permitted for fiscal years beginning after December 15, 2020, including interim periods within those fiscal years. We are continuing to evaluate the impact of this update to our consolidated financial statements and would expect to adopt at the required adoption date of January 1, 2022.

BASIS OF PRESENTATION AND SIG_3

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Equity Method Investments	Investees and equity ownership percentages are presented below: Investee Equity Ownership Percentage Combined Metals of Chicago, LLC 40.0% Spartan Steel Coating, LLC 48.0%
Depreciation Disclosure	Depreciation and depletion are recorded over the following estimated useful lives: Asset Class Basis Life Land, land improvements and mineral rights Land and mineral rights Units of production Life of mine Land improvements Straight line 20 to 45 years Buildings Straight line 20 to 45 years Equipment Straight line/Double declining balance 3 to 20 years The following table indicates the carrying value of each of the major classes of our depreciable assets: (In Millions) December 31, 2020 2019 Land, land improvements, and mineral rights $ 1,213 $ 582 Buildings 703 158 Equipment 6,786 1,456 Other 151 101 Construction in progress 1,364 730 Total property, plant and equipment 1 10,217 3,027 Allowance for depreciation and depletion (1,474) (1,098) Property, plant, and equipment, net $ 8,743 $ 1,929 1 Includes right-of-use assets related to finance leases of $361 million and $49 million as of December 31, 2020 and 2019, respectively. The net book value of the mineral and land rights are as follows: (In Millions) December 31, 2020 2019 Mineral rights: Cost $ 773 $ 537 Depletion (142) (134) Net mineral rights $ 631 $ 403 Land rights $ 361 $ 12
Schedule of Finite-Lived Intangible Assets	Intangible assets and liabilities are subject to periodic amortization on a straight-line basis over their estimated useful lives as follows: Type Basis Useful Life Intangible assets: Customer relationships Straight line 18 years Developed technology Straight line 17 years Trade names and trademarks Straight line 10 years Mining permits Straight line Life of mine Intangible liabilities: Above-market supply contracts Straight line Contract term

SUPPLEMENTARY FINANCIAL STATE_2

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Text Block [Abstract]
Accounts Receivable, Allowance for Credit Loss	The following is a roll-forward of our allowance for credit losses associated with Accounts receivable, net : (In Millions) 2020 2019 Allowance for credit losses as of January 1 $ — $ — Increase in allowance (5) — Allowance for credit losses as of December 31 $ (5) $ —
Schedule Of Inventories	The following table presents the detail of our Inventories in the Statements of Consolidated Financial Position: (In Millions) Year Ended December 31, 2020 2019 Product inventories Finished and semi-finished goods $ 2,125 $ 114 Work-in-process — 69 Raw materials 1,431 9 Total product inventories 3,556 192 Manufacturing supplies and critical spares 272 125 Inventories $ 3,828 $ 317
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	A reconciliation of capital additions to cash paid for capital expenditures is as follows: (In Millions) Year Ended December 31, 2020 2019 2018 Capital additions $ 483 $ 690 $ 395 Less: Non-cash accruals (86) 15 94 Right-of-use assets - finance leases 44 29 8 Grants — (10) (3) Cash paid for capital expenditures including deposits $ 525 $ 656 $ 296 Cash payments (receipts) for interest and income taxes are as follows: (In Millions) 2020 2019 2018 Taxes paid on income $ 5 $ — $ 3 Income tax refunds (120) (118) (11) Interest paid on debt obligations net of capitalized interest 1 170 98 106 1 Capitalized interest was $53 million, $25 million and $7 million for the years ended December 31, 2020, 2019 and 2018, respectively.
Schedule of Noncash or Part Noncash Acquisitions	Non-Cash Investing and Financing Activities (In Millions) 2020 2019 2018 Fair value of common shares issued as part of consideration in connection with AM USA Transaction $ 990 $ — $ — Fair value of Series B Participating Redeemable Preferred Stock issued as part of consideration in connection with AM USA Transaction 738 — — Fair value of settlement of a pre-existing relationship as part of consideration in connection with AM USA Transaction 237 — — Fair value of common shares issued as consideration in connection with AK Steel Merger 618 — — Fair value of equity awards assumed in connection with AK Steel Merger 4 — —

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
Schedule of Business Acquisitions, by Acquisition	The fair value of the total purchase consideration was determined as follows: (In Millions) Fair value of Cliffs common shares issued $ 990 Fair value of Series B Participating Redeemable Preferred Stock issued 738 Fair value of settlement of a pre-existing relationship 237 Cash consideration (subject to customary working capital adjustments) 631 Total purchase consideration $ 2,596 The fair value of Cliffs common shares issued is calculated as follows: Number of Cliffs common shares issued 78,186,671 Closing price of Cliffs common share as of December 9, 2020 $ 12.66 Fair value of Cliffs common shares issued (in millions) $ 990 The fair value of Cliffs Series B Participating Redeemable Preferred Stock issued is calculated as follows: Number of Cliffs Series B Participating Redeemable Preferred Stock issued 583,273 Redemption price as of December 9, 2020 $ 1,266 Fair value of Cliffs Series B Participating Redeemable Preferred Stock issued (in millions) $ 738 The fair value of the estimated cash consideration is comprised of the following: (In Millions) Cash consideration pursuant to the AM USA Transaction Agreement $ 505 Cash consideration for purchase of the remaining JV partners' interest of Kote and Tek 182 Estimated total cash consideration receivable (56) Total estimated cash consideration $ 631 The cash portion of the purchase price is subject to customary working capital adjustments. The fair value of the settlement of a pre-existing relationship is comprised of the following: (In Millions) Accounts receivable $ 97 Freestanding derivative asset from customer supply agreement 140 Total fair value of settlement of a pre-existing relationship $ 237 The fair value of the total purchase consideration was determined as follows: (In Millions) Fair value of AK Steel debt $ 914 Fair value of Cliffs common shares issued for AK Steel outstanding common stock 618 Other 3 Total purchase consideration $ 1,535 The fair value of AK Steel's debt included in the consideration is calculated as follows: (In Millions) Credit Facility $ 590 7.50% Senior Secured Notes due July 2023 324 Fair value of debt included in consideration $ 914
Schedule of Business Acquisitions by Acquisition, Equity Interest Issued or Issuable	The fair value of Cliffs common shares issued for outstanding shares of AK Steel common stock and with respect to Cliffs common shares underlying converted AK Steel equity awards that vested upon completion of the AK Steel Merger is calculated as follows: (In Millions, Except Per Share Amounts) Number of shares of AK Steel common stock issued and outstanding 317 Exchange ratio 0.400 Shares of Cliffs common shares issued to AK Steel stockholders 127 Price per share of Cliffs common shares $ 4.87 Fair value of Cliffs common shares issued for outstanding AK Steel common stock $ 618
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The preliminary purchase price allocation to assets acquired and liabilities assumed in the AM USA Transaction was: (In Millions) Cash and cash equivalents $ 35 Accounts receivable, net 349 Inventories 2,115 Income tax receivable, current 12 Other current assets 22 Property, plant and equipment 4,017 Other non-current assets 158 Accounts payable (758) Accrued employment costs (271) State and local taxes (76) Pension and OPEB liabilities, current (109) Other current liabilities (322) Pension and OPEB liabilities, non-current (3,195) Other non-current liabilities (598) Noncontrolling interest (13) Net identifiable assets acquired 1,366 Goodwill 1,230 Total net assets acquired $ 2,596 The preliminary purchase price allocation to assets acquired and liabilities assumed in the AK Steel Merger was: (In Millions) Initial Allocation of Consideration Measurement Period Adjustments Updated Allocation Cash and cash equivalents $ 38 $ 1 $ 39 Accounts receivable, net 666 (2) 664 Inventories 1,563 (243) 1,320 Income tax receivable, current 3 — 3 Other current assets 65 (16) 49 Property, plant and equipment 2,184 128 2,312 Deferred income taxes — 30 30 Other non-current assets 475 (3) 472 Accounts payable (636) (8) (644) Accrued employment costs (94) 1 (93) State and local taxes (35) 4 (31) Pension and OPEB liabilities, current (75) (3) (78) Other current liabilities (201) 5 (196) Long-term debt (1,179) — (1,179) Pension and OPEB liabilities, non-current (873) 2 (871) Other non-current liabilities (507) 72 (435) Noncontrolling interest — (1) (1) Net identifiable assets acquired 1,394 (33) 1,361 Goodwill 141 33 174 Total net assets acquired $ 1,535 $ — $ 1,535
Finite-Lived and Indefinite-Lived Intangible Assets Acquired as Part of Business Combination	The preliminary purchase price allocated to identifiable intangible assets and liabilities acquired was: (In Millions) Weighted Average Life (In Years) Intangible assets: Customer relationships $ 77 18 Developed technology 60 17 Trade names and trademarks 11 10 Total identifiable intangible assets $ 148 17 Intangible liabilities: Above-market supply contracts $ (71) 12
Business Acquisition, Pro Forma Information	The following table provides unaudited pro forma financial information, prepared in accordance with Topic 805, for the years ended December 31, 2020 and 2019, as if ArcelorMittal USA and AK Steel had been acquired as of January 1, 2019: (In Millions) Year Ended December 31, 2020 2019 Revenues $ 12,837 $ 17,163 Net income (loss) attributable to Cliffs shareholders (520) (11)

REVENUES (Tables)

REVENUES (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenues from Product Sales and Services [Table Text Block]	The following table represents our Revenues by market: (In Millions) Year Ended December 31, 2020 2019 2018 Steelmaking: Automotive $ 2,062 $ — $ — Infrastructure and manufacturing 784 — — Distributors and converters 696 — — Steel producers 1 1,423 1,990 2,332 Total steelmaking 4,965 1,990 2,332 Other Businesses: Automotive 329 — — Infrastructure and manufacturing 34 — — Distributors and converters 26 — — Total Other Businesses 389 — — Total revenues $ 5,354 $ 1,990 $ 2,332 1 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020. The following table represents our consolidated Revenues by product line: (Dollars In Millions, Sales Volumes in Thousands) Year Ended December 31, 2020 2019 2018 Revenue Volume 1 Revenue Volume 1 Revenue Volume 1 Steelmaking: Hot-rolled steel $ 386 633 $ — — $ — — Cold-rolled steel 490 682 — — — — Coated steel 1,747 1,911 — — — — Stainless and electrical steel 868 416 — — — — Other steel products 92 141 — — — — Iron products 2 1,335 11,707 1,990 18,583 2,332 20,563 Other 47 N/A — — — — Total steelmaking 4,965 1,990 2,332 Other Businesses: Other 389 N/A — N/A — N/A Total revenues $ 5,354 $ 1,990 $ 2,332 1 Carbon steel products, stainless and electrical steel and plate steel volumes are stated in net tons. Iron product volumes are stated in long tons. 2 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020.
Deferred Revenue [Table Text Block]	The table below summarizes our deferred revenue balances: (In Millions) Deferred Revenue (Current) Deferred Revenue (Long-Term) 2020 2019 2020 2019 Opening balance as of January 1 $ 22 $ 21 $ 26 $ 39 Net increase (decrease) (15) 1 (26) (13) Closing balance as of December 31 $ 7 $ 22 $ — $ 26

SEGMENT REPORTING (Tables)

SEGMENT REPORTING (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information, By Segment	Our results by segment are as follows: (In Millions) Year Ended December 31, 2020 2019 2018 Revenues: Steelmaking 1 $ 4,965 $ 1,990 $ 2,332 Other Businesses 389 — — Total revenues $ 5,354 $ 1,990 $ 2,332 Adjusted EBITDA: Steelmaking $ 433 $ 636 $ 872 Other Businesses 47 — — Corporate and eliminations (127) (111) (106) Total Adjusted EBITDA $ 353 $ 525 $ 766 1 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020. The following table provides a reconciliation of our consolidated Net income (loss) to total Adjusted EBITDA: (In Millions) Year Ended December 31, 2020 2019 2018 Net income (loss) $ (81) $ 293 $ 1,128 Less: Interest expense, net (238) (101) (121) Income tax benefit (expense) 111 (18) 460 Depreciation, depletion and amortization (308) (85) (89) 354 497 878 Less: EBITDA from noncontrolling interests 1 56 — — Gain (loss) on extinguishment of debt 130 (18) (7) Severance costs (38) (2) — Acquisition-related costs excluding severance costs (52) (7) — Amortization of inventory step-up (96) — — Impact of discontinued operations 1 (1) 121 Foreign exchange remeasurement — — (1) Impairment of other long-lived assets — — (1) Total Adjusted EBITDA $ 353 $ 525 $ 766 1 EBITDA of noncontrolling interests includes $41 million for income and $15 million for depreciation, depletion and amortization for the year ended December 31, 2020. The following table summarizes our depreciation, depletion and amortization and capital additions by segment: (In Millions) Year Ended December 31, 2020 2019 2018 Depreciation, depletion and amortization: Steelmaking $ 276 $ 80 $ 68 Other Businesses 27 — — Corporate 5 5 6 Total depreciation, depletion and amortization $ 308 $ 85 $ 74 Capital additions 1: Steelmaking $ 436 $ 687 $ 393 Other Businesses 45 — — Corporate 2 3 2 Total capital additions $ 483 $ 690 $ 395 1 Refer to NOTE 2 - SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION for additional information.
Summary of Assets by Segment	The following summarizes our assets by segment: (In Millions) December 31, 2020 2019 Assets: Steelmaking $ 15,849 $ 2,557 Other Businesses 239 — Total segment assets 16,088 2,557 Corporate 683 947 Total assets $ 16,771 $ 3,504
Schedule of Revenue from External Customers and Long-Lived Assets, by Geographical Areas	Included in the consolidated financial statements are the following amounts relating to geographic location based on product destination: (In Millions) 2020 2019 2018 Revenues: United States $ 4,580 $ 1,505 $ 1,847 Canada 602 448 395 Other countries 172 37 90 Total revenues $ 5,354 $ 1,990 $ 2,332 Property, plant and equipment, net: United States $ 8,647 $ 1,929 $ 1,286 Canada 91 — — Other countries 5 — — Total property, plant and equipment, net $ 8,743 $ 1,929 $ 1,286

PROPERTY, PLANT AND EQUIPMENT (

PROPERTY, PLANT AND EQUIPMENT (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property Plant and Equipment [Table Text Block]	Depreciation and depletion are recorded over the following estimated useful lives: Asset Class Basis Life Land, land improvements and mineral rights Land and mineral rights Units of production Life of mine Land improvements Straight line 20 to 45 years Buildings Straight line 20 to 45 years Equipment Straight line/Double declining balance 3 to 20 years The following table indicates the carrying value of each of the major classes of our depreciable assets: (In Millions) December 31, 2020 2019 Land, land improvements, and mineral rights $ 1,213 $ 582 Buildings 703 158 Equipment 6,786 1,456 Other 151 101 Construction in progress 1,364 730 Total property, plant and equipment 1 10,217 3,027 Allowance for depreciation and depletion (1,474) (1,098) Property, plant, and equipment, net $ 8,743 $ 1,929 1 Includes right-of-use assets related to finance leases of $361 million and $49 million as of December 31, 2020 and 2019, respectively. The net book value of the mineral and land rights are as follows: (In Millions) December 31, 2020 2019 Mineral rights: Cost $ 773 $ 537 Depletion (142) (134) Net mineral rights $ 631 $ 403 Land rights $ 361 $ 12

GOODWILL AND INTANGIBLE ASSET_2

GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The following is a summary of goodwill by segment: (In Millions) December 31, 2020 2019 Steelmaking $ 1,232 $ 2 Other Businesses 174 — Total goodwill $ 1,406 $ 2
Schedule of Acquired Intangible Assets and Liabilities by Major Class	The following is a summary of our intangible assets and liabilities: (In Millions) Classification 1 Gross Amount Accumulated Amortization Net Amount As of December 31, 2020 Intangible assets: Customer relationships Other non-current assets $ 77 $ (3) $ 74 Technology Other non-current assets 60 (3) 57 Trade names and trademarks Other non-current assets 11 (1) 10 Mining permits Other non-current assets 72 (25) 47 Total intangible assets $ 220 $ (32) $ 188 Intangible liabilities: Above-market supply contracts Other non-current liabilities $ (71) $ 7 $ (64) As of December 31, 2019 Intangible assets: Mining permits Other non-current assets $ 72 $ (24) $ 48 1 Amortization of intangible liabilities related to above-market supply contracts and intangible assets related to mining permits is recognized in Cost of goods sold . Amortization of all other intangible assets is recognized in Selling, general and administrative expenses.

DEBT AND CREDIT FACILITIES (Tab

DEBT AND CREDIT FACILITIES (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule Of Long-Term Debt	The following represents a summary of our long-term debt: (In Millions) December 31, 2020 Debt Instrument Issuer 1 Annual Effective Interest Rate Total Principal Amount Unamortized Debt Issuance Costs Unamortized Premiums (Discounts) Total Debt Senior Secured Notes: 4.875% 2024 Senior Secured Notes Cliffs 5.00% $ 395 $ (3) $ (1) $ 391 9.875% 2025 Senior Secured Notes Cliffs 10.57% 955 (8) (25) 922 6.75% 2026 Senior Secured Notes Cliffs 6.99% 845 (20) (9) 816 Senior Unsecured Notes: 7.625% 2021 AK Senior Notes AK Steel 7.33% 34 — — 34 7.50% 2023 AK Senior Notes AK Steel 6.17% 13 — — 13 6.375% 2025 Senior Notes Cliffs 8.11% 64 — (4) 60 6.375% 2025 AK Senior Notes AK Steel 8.11% 29 — (2) 27 1.50% 2025 Convertible Senior Notes Cliffs 6.26% 296 (4) (49) 243 5.75% 2025 Senior Notes Cliffs 6.01% 396 (3) (4) 389 7.00% 2027 Senior Notes Cliffs 9.24% 73 — (8) 65 7.00% 2027 AK Senior Notes AK Steel 9.24% 56 — (6) 50 5.875% 2027 Senior Notes Cliffs 6.49% 556 (4) (18) 534 6.25% 2040 Senior Notes Cliffs 6.34% 263 (2) (3) 258 IRBs due 2024 to 2028 AK Steel Various 92 — 2 94 EDC Revolving Facility 3 * 3.25% 40 — — 18 ABL Facility 3 Cliffs 2 2.15% 3,500 — — 1,510 Total debt 5,424 Less: current debt 34 Total long-term debt $ 5,390 * Our subsidiaries, Fleetwood Metal Industries Inc. and The Electromac Group Inc., are the borrowers under the EDC Revolving Facility. 1 Unless otherwise noted, references in this column and throughout this Note 8 - DEBT AND CREDIT FACILITIES to "Cliffs" are to Cleveland-Cliffs Inc., and references to "AK Steel" are to AK Steel Corporation (n/k/a Cleveland-Cliffs Steel Corporation). 2 Refers to Cleveland-Cliffs Inc. as borrower under our ABL Facility. 3 The total principal amounts for the indicated credit facilities are stated at their respective maximum borrowing capacities. (In Millions) December 31, 2019 Debt Instrument Issuer 1 Annual Effective Interest Rate Total Principal Amount Debt Issuance Costs Unamortized Discounts Total Debt Senior Secured Notes: 4.875% 2024 Senior Notes Cliffs 5.00% $ 400 $ (5) $ (2) $ 393 Senior Unsecured Notes: 1.50% 2025 Convertible Senior Notes Cliffs 6.26% 316 (4) (65) 247 5.75% 2025 Senior Notes Cliffs 6.01% 473 (3) (6) 464 5.875% 2027 Senior Notes Cliffs 6.49% 750 (6) (27) 717 6.25% 2040 Senior Notes Cliffs 6.34% 298 (2) (3) 293 Former ABL Facility Cliffs 2 N/A 450 N/A N/A — Total long-term debt $ 2,114 1 Unless otherwise noted, references in this column to "Cliffs" are to Cleveland-Cliffs Inc. 2 Refers to Cleveland-Cliffs Inc. and certain of its subsidiaries as borrowers under our Former ABL Facility.
Debt Instrument Redemption	The following is a summary of redemption prices for our 9.875% 2025 Senior Secured Notes: Redemption Period Redemption Price 1 Restricted Amount Prior to October 17, 2022 - using proceeds of equity issuance 109.875 % Up to 35% of original aggregate principal Prior to October 17, 2022 2 100.000 Beginning on October 17, 2022 107.406 Beginning on April 17, 2023 104.938 Beginning on April 17, 2024 102.469 Beginning on April 17, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to, but excluding, the redemption date. 2 Plus a "make-whole" premium. Redemption Period Redemption Price 1 Restricted Amount Prior to March 15, 2022 - using proceeds of equity issuance 106.750 % Up to 35% of original aggregate principal Prior to March 15, 2022 2 100.000 Beginning on March 15, 2022 105.063 Beginning on March 15, 2023 103.375 Beginning on March 15, 2024 101.688 Beginning on March 15, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to, but excluding, the redemption date. 2 Plus a "make-whole" premium. Redemption Period Redemption Price 1 Beginning on January 15, 2021 102.438 % Beginning on January 15, 2022 101.219 Beginning on January 15, 2023 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. Redemption Period Redemption Price 1 Beginning on October 15, 2020 103.188 % Beginning on October 15, 2021 101.594 Beginning on October 15, 2022 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. Redemption Period Redemption Price 1 Prior to March 15, 2022 2 100.000 % Beginning on March 15, 2022 103.500 Beginning on March 15, 2023 102.333 Beginning on March 15, 2024 101.167 Beginning on March 15, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. 2 Plus a "make-whole" premium. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for the 7.50% 2023 AK Senior Notes: Redemption Period Redemption Price 1 Beginning on July 15, 2020 101.875 % Beginning on July 15, 2021 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for the 6.375% 2025 AK Senior Notes: Redemption Period Redemption Price 1 Beginning on October 15, 2020 103.188 % Beginning on October 15, 2021 101.594 Beginning on October 15, 2022 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. The following is a summary of redemption prices (expressed as a percentage of the principal amount to be redeemed) for the 7.00% 2027 AK Senior Notes: Redemption Period Redemption Price 1 Prior to March 15, 2022 2 100.000 % Beginning on March 15, 2022 103.500 Beginning on March 15, 2023 102.333 Beginning on March 15, 2024 101.167 Beginning on March 15, 2025 and thereafter 100.000 1 Plus accrued and unpaid interest, if any, up to but excluding the redemption date. 2 Plus a "make-whole" premium.
Schedule of Extinguishment of Debt [Table Text Block]	The following is a summary of the debt extinguished and the respective impact on extinguishment: (In Millions) Year Ended Debt Extinguished Gain (Loss) on Extinguishment 7.625% 2021 AK Senior Notes $ 373 $ — 7.50% 2023 AK Senior Notes 367 3 4.875% 2024 Senior Secured Notes 6 1 6.375% 2025 Senior Notes 168 21 1.50% 2025 Convertible Senior Notes 20 1 5.75% 2025 Senior Notes 77 16 7.00% 2027 Senior Notes 262 27 5.875% 2027 Senior Notes 195 49 6.25% 2040 Senior Notes 36 13 6.375% 2025 AK Senior Notes 9 (1) Total $ 1,513 $ 130 The following is a summary of the debt extinguished and the respective impact on extinguishment: (In Millions) Year Ended Debt Extinguished (Loss) on Extinguishment 4.875% 2021 Senior Notes $ 124 $ (5) 5.75% 2025 Senior Notes 600 (13) Total $ 724 $ (18) The following is a summary of the debt extinguished and the respective impact on extinguishment: (In Millions) Year Ended Debt Extinguished (Loss) on Extinguishment 5.90% 2020 Senior Notes $ 89 $ (3) 4.80% 2020 Senior Notes 122 (4) 4.875% 2021 Senior Notes 14 — 5.75% 2025 Senior Notes 2 — Total $ 227 $ (7)
Schedule of Line of Credit Facilities [Table Text Block]	The following represents a summary of our borrowing capacity under the ABL Facility: (In Millions) December 31, Available borrowing base on ABL Facility 1 $ 3,500 Borrowings (1,510) Letter of credit obligations 2 (247) Borrowing capacity available $ 1,743 1 As of December 31, 2020, the ABL Facility has a maximum borrowing base of $3.5 billion. The available borrowing base is determined by applying customary advance rates to eligible accounts receivable, inventory and certain mobile equipment. 2 We issued standby letters of credit with certain financial institutions in order to support business obligations including, but not limited to, workers' compensation, employee severance, insurance, operating agreements, IRBs and environmental obligations.
Schedule of Maturities of Long-term Debt [Table Text Block]	The following represents a summary of our debt instrument maturities based on the principal amounts outstanding at December 31, 2020: (In Millions) Maturities of Debt 2021 $ 34 2022 — 2023 13 2024 457 2025 1,740 Thereafter 3,351 Total maturities of debt $ 5,595

FAIR VALUE OF FINANCIAL INSTR_2

FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value, by Balance Sheet Grouping [Table Text Block]	The following represents the assets and liabilities measured at fair value as of December 31, 2019: (In Millions) December 31, 2019 Quoted Prices in Active Significant Other Observable Inputs Significant Unobservable Inputs Total Assets: Cash equivalents - Commercial paper $ — $ 188 $ — $ 188 Other current assets: Customer supply agreement — — 45 45 Total $ — $ 188 $ 45 $ 233
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation	The following tables represent a reconciliation of the changes in fair value of financial instruments measured at fair value on a recurring basis using significant unobservable inputs (Level 3): (In Millions) Level 3 Assets 2020 2019 Beginning balance - January 1 $ 45 $ 89 Total gains included in earnings 122 79 Settlements (27) (123) Settlement of pre-existing relationship (140) — Ending balance - December 31 $ — $ 45 Total gains for the period included in earnings attributable to the change in unrealized gains on assets still held at the reporting date $ — $ 45
Schedule Of Carrying Value And Fair Value Of Financial Instruments	A summary of the carrying value and fair value of other financial instruments were as follows: (In Millions) December 31, 2020 December 31, 2019 Classification Carrying Fair Value Carrying Fair Value Senior notes Level 1 $ 3,802 $ 4,446 $ 2,114 $ 2,237 IRBs due 2024 to 2028 Level 1 94 91 — — EDC Revolving Facility - outstanding balance Level 2 18 18 — — ABL Facility - outstanding balance Level 2 1,510 1,510 — — Total $ 5,424 $ 6,065 $ 2,114 $ 2,237

PENSIONS AND OTHER POSTRETIRE_2

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Tables)	12 Months Ended
	Dec. 31, 2020
Postemployment Benefits [Abstract]
Schedule of Defined Benefit Plans Disclosures	The following tables and information provide additional disclosures: (In Millions) Pension Benefits OPEB Change in benefit obligations: 2020 2019 2020 2019 Benefit obligations — beginning of year $ 1,021 $ 906 $ 255 $ 242 Service cost 23 17 8 2 Interest cost 64 35 19 10 Actuarial loss 162 112 14 19 Benefits paid (146) (62) (89) (26) Participant contributions — — 22 6 Acquired through business combinations 5,535 — 3,528 — Effect of settlement (94) — — — Other — 13 — 2 Benefit obligations — end of year $ 6,565 $ 1,021 $ 3,757 $ 255 Change in plan assets: Fair value of plan assets — beginning of year $ 749 $ 687 $ 260 $ 240 Actual return on plan assets 472 98 45 35 Participant contributions — — 17 1 Employer contributions 50 16 30 3 Benefits paid (146) (62) (88) (19) Acquired through business combinations 4,301 — 519 — Effect of settlement (94) — — — Other — 10 — — Fair value of plan assets — end of year $ 5,332 $ 749 $ 783 $ 260 Funded status $ (1,233) $ (272) $ (2,974) $ 5 Amounts recognized in Statements of Financial Position: Non-current assets $ 3 $ — $ 54 $ 49 Current liabilities (12) — (139) (4) Non-current liabilities (1,224) (272) (2,889) (40) Total amount recognized $ (1,233) $ (272) $ (2,974) $ 5 Amounts recognized in accumulated other comprehensive loss: Net actuarial loss $ 164 $ 382 $ 56 $ 73 Prior service cost (credit) 6 7 (6) (8) Net amount recognized $ 170 $ 389 $ 50 $ 65 Annual contributions to the pension plans are made within income tax deductibility restrictions in accordance with statutory regulations. OPEB plans are not subject to minimum regulatory funding requirements, but rather amounts are contributed pursuant to bargaining agreements. (In Millions) Pension Other Benefits Company Contributions (Reimbursements) VEBA Direct Total 2019 $ 16 $ — $ 4 $ 4 2020 50 — 25 25 2021 (Expected) 1 202 (16) 144 128 1 Pursuant to the applicable bargaining agreements, benefits can be paid from certain VEBAs that are at least 70% funded (all VEBAs are over 70% funded at December 31, 2020). Certain agreements with plans holding VEBA assets have capped healthcare costs. For the ArcelorMittal USA VEBA, depending on funding levels and/or Company profits, we may withdraw money from the VEBA plans to the extent funds are available for costs in excess of the cap. The 2021 expected pension contributions include $118 million in deferred 2020 pension contributions in connection with the CARES Act that were paid on January 4, 2021.
Components Of Net Periodic Benefit Cost	Components of Net Periodic Benefit Cost (Credit) (In Millions) Pension Benefits OPEB 2020 2019 2018 2020 2019 2018 Service cost $ 23 $ 17 $ 19 $ 8 $ 2 $ 2 Interest cost 64 35 30 19 10 8 Expected return on plan assets (140) (55) (60) (20) (17) (18) Amortization: Net actuarial loss 27 24 21 3 5 5 Prior service costs (credits) 1 1 2 (2) (2) (3) Settlements (6) — 1 — — — Net periodic benefit cost (credit) $ (31) $ 22 $ 13 $ 8 $ (2) $ (6) For 2021, we estimate net periodic benefit cost (credit) as follows: (In Millions) Defined benefit pension plans $ (168) OPEB plans 86 Total $ (82)
Components of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following includes details on the significant actuarial losses (gains) impacting the benefit obligation: (In Millions) Pension Benefits OPEB 2020 2019 2020 2019 Discount rates $ 181 $ 106 $ 44 $ 26 Demographic (gains) losses (3) 12 (11) 4 Mortality (16) (6) (4) (4) Per capita claims — — (10) (9) Other — — (5) 3 Actuarial loss on benefit obligation 162 112 14 20 Actual returns on assets over expected (332) (44) (26) (18) Amortization of net actuarial loss (27) (24) (3) (5) Amortization of prior service credits (costs) (1) (1) 2 2 Settlements 6 — — — Other (27) 7 (2) (5) Total recognized in accumulated other comprehensive loss (income) $ (219) $ 50 $ (15) $ (6)
Estimated Future Benefit Payments	Estimated Future Benefit Payments (In Millions) Pension OPEB 2021 $ 486 $ 191 2022 462 185 2023 480 180 2024 455 178 2025 433 176 2026-2030 1,983 884
Defined Benefit Plan, Assumptions	The following represents weighted-average assumptions used to determine benefit obligations: Pension Benefits OPEB December 31, December 31, 2020 2019 2020 2019 Discount rate 2.34 % 3.27 % 2.71 % 3.28 % Interest crediting rate 5.25 6.00 N/A N/A Compensation rate increase 2.56 2.53 3.00 3.00 The following represents weighted-average assumptions used to determine net benefit cost: Pension Benefits OPEB December 31, December 31, 2020 2019 2018 2020 2019 2018 Obligation discount rate 3.02 % 4.27 % 3.58 % 3.28 % 4.29 % 3.60 % Service cost discount rate 3.34 4.35 3.64 3.35 4.49 3.73 Interest cost discount rate 2.53 3.92 3.16 2.51 3.94 3.11 Interest crediting rate 5.50 6.00 6.00 N/A N/A N/A Expected return on plan assets 7.69 8.25 8.25 6.82 7.00 7.00 Compensation rate increase 2.56 2.53 2.49 3.00 3.00 3.00
Assumed Health Care Cost Trend Rates	The following represents assumed weighted-average health care cost trend rates: December 31, 2020 2019 Health care cost trend rate assumed for next year 6.05 % 6.50 % Ultimate health care cost trend rate 4.59 5.00 Year that the ultimate rate is reached 2031 2026
Plan Assets and Asset Allocation	The following table reflects the actual asset allocations for pension and VEBA assets as of December 31, 2020 and 2019, as well as the 2021 weighted average target asset allocations. Equity investments include securities in large-cap, mid-cap and small-cap companies located in the U.S. and worldwide. Fixed income investments primarily include corporate bonds and government debt securities. Pension Assets VEBA Assets Asset Category 2021 Percentage of 2021 Percentage of 2020 2019 2020 2019 Equity securities 41.3 % 51.8 % 44.0 % 20.3 % 22.2 % 7.2 % Fixed income 39.7 33.8 27.6 69.6 66.4 79.8 Hedge funds 5.0 2.2 5.4 1.1 1.8 4.8 Private equity 2.2 2.1 6.6 1.4 0.4 0.7 Structured credit 5.2 5.0 7.0 1.0 0.9 2.1 Real estate 5.2 3.3 9.4 1.1 1.8 5.4 Absolute return fixed income 1.4 1.8 — 5.5 6.5 — Total 100.0 % 100.0 % 100.0 % 100.0 % 100.0 % 100.0 %
Defined Benefit Plan, Plan Assets, Category [Table Text Block]	The fair value of our pension assets by asset category is as follows: (In Millions) Quoted Prices in Active Significant Other Significant Investments Measured at Net Asset Value Total Asset Category 2020 2019 2020 2019 2020 2019 2020 2019 2020 2019 Equity securities: U.S. equities $ 1,163 $ 169 $ — $ — $ — $ — $ 787 $ — $ 1,950 $ 169 Global equities 615 161 — — — — 195 — 810 161 Fixed income: U.S. government securities 1 141 11 295 22 — — 40 — 476 33 U.S. corporate bonds 512 174 466 — — — 303 — 1,281 174 Non U.S. and other bonds — — 46 — — — — — 46 — Hedge funds — — — — 118 40 — — 118 40 Private equity — — — — 114 50 — — 114 50 Structured credit — — — — 264 52 — — 264 52 Real estate — — — — 174 70 — — 174 70 Absolute return fixed income — — — — — — 99 — 99 — Total $ 2,431 $ 515 $ 807 $ 22 $ 670 $ 212 $ 1,424 $ — $ 5,332 $ 749 1 Includes cash equivalents. (In Millions) Quoted Prices in Active Significant Other Significant Investments Measured at Net Asset Value Total Asset Category 2020 2019 2020 2019 2020 2019 2020 2019 2020 2019 Equity securities: U.S. equities $ 26 $ 12 $ — $ — $ — $ — $ 93 $ — $ 119 $ 12 Global equities 6 7 — — — — 49 — 55 7 Fixed income: U.S. government securities 1 62 — 94 — — — — — 156 — U.S. corporate bonds 237 166 127 41 — — — — 364 207 Hedge funds — — — — 14 12 — — 14 12 Private equity — — — — 3 2 — — 3 2 Structured credit — — — — 7 6 — — 7 6 Real estate — — — — 14 14 — — 14 14 Absolute return fixed income — — — — — — 51 — 51 — Total $ 331 $ 185 $ 221 $ 41 $ 38 $ 34 $ 193 $ — $ 783 $ 260 1 Includes cash equivalents.
Effect of Fair Value Measurements Using Significant Unobservable Inputs on Changes in Plan Assets	The following represents the fair value measurements of changes in plan assets using significant unobservable inputs (Level 3): (In Millions) Pension Assets VEBA Assets 2020 2019 2020 2019 Beginning balance — January 1 $ 212 $ 229 $ 34 $ 36 Actual return on plan assets: Relating to assets still held at the reporting date 8 (1) 2 1 Relating to assets sold during the period 6 30 1 — Purchases 195 17 — — Sales (13) (60) (1) (3) Acquired through business combinations 262 — 2 — Other — (3) — — Ending balance — December 31 $ 670 $ 212 $ 38 $ 34
Multiemployer Plan	Information with respect to multiemployer plans in which we participate follows: Pension Fund EIN/Pension Plan Number Pension Protection Act Zone Status (a) FIP/RP Status Pending/Implemented (b) Contributions Surcharge Imposed (c) Expiration Date of Collective Bargaining Agreement 2020 2019 2020 2019 2018 Steelworkers Pension Trust 23-6648508/499 Green Green No $ 14 $ 4 $ 4 No 1/22/2021 to 10/1/2022 IAM National Pension Fund’s National Pension Plan 51-6031295/002 Red Green Yes 16 — — Yes 5/31/2022 to 5/15/2023 American Maritime Officers Plan 13-1936709/001 Green Green No — — — No 7/31/2021 Total $ 30 $ 4 $ 4 (a) The most recent Pension Protection Act zone status available in 2020 and 2019 is for each plan's year-end at December 31, 2019 and 2018. The plan's actuary certifies the zone status. Generally, plans in the red zone are less than 65% funded, plans in the yellow zone are between 65% and 80% funded, and plans in the green zone are at least 80% funded. (b) The "FIP/RP Status Pending/Implemented" column indicates plans for which a financial improvement plan or a rehabilitation plan is either pending or has been implemented, as defined by ERISA. (c) The surcharge represents an additional required contribution due as a result of the critical funding status of the plan.

STOCK COMPENSATION PLANS (Table

STOCK COMPENSATION PLANS (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Share-based Payment Arrangement, Cost by Plan [Table Text Block]	The following table summarizes the total compensation expense recognized for stock-based compensation awards: (In Millions, except per Year Ended December 31, 2020 2019 2018 Cost of goods sold $ (2) $ (2) $ (2) Selling, general and administrative expenses (13) (16) (13) Acquisition-related costs (2) — — Stock based compensation expense (17) (18) (15) Income tax benefit 1 4 4 — Stock based compensation expense, net of tax $ (13) $ (14) $ (15) Decrease in basic earnings per common share $ (0.03) $ (0.05) $ (0.05) Decrease in diluted earnings per common share $ (0.03) $ (0.05) $ (0.05) 1 No income tax benefit in 2018 due to the full valuation allowance.
Share-based Payment Arrangement, Performance Shares, Activity	The following table summarizes the performance award activity: Year Ended December 31, 2020 2019 2018 Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Outstanding at beginning of year 1,935,878 $ 15.58 1,424,723 $ 14.46 1,848,312 $ 13.42 Granted 2,510,853 5.49 572,104 18.31 675,599 11.93 Distributed (1,938,786) 12.23 — — (489,953) 10.89 Performance adjustment 549,154 15.63 — — (560,720) 10.69 Forfeited/canceled (604,873) 5.70 (60,949) 15.12 (48,515) 19.33 Outstanding at end of year 2,452,226 $ 10.34 1,935,878 $ 15.58 1,424,723 $ 14.46
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity	The following table summarizes the restricted stock units activity: Year Ended December 31, 2020 2019 2018 Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Outstanding at beginning of year 2,147,183 $ 9.10 4,694,360 $ 4.18 4,403,112 $ 3.64 Granted 1,160,928 4.87 572,104 11.24 685,599 7.53 Distributed (1,101,115) 8.58 (3,058,307) 1.95 (254,196) 3.30 Forfeited/canceled (63,413) 7.31 (60,974) 9.31 (140,155) 5.17 Outstanding at end of year 2,143,583 $ 7.12 2,147,183 $ 9.10 4,694,360 $ 4.18
Share-based Payment Arrangement, Option, Activity	The following table summarizes the stock option activity: Year Ended December 31, 2020 2019 2018 Number of Shares Weighted Average Exercise Price Number of Shares Weighted Average Exercise Price Number of Shares Weighted Average Excercise Price Outstanding at beginning of year 563,230 $ 10.42 563,230 $ 10.42 599,870 $ 10.25 Granted 2,010,277 11.86 — — — — Exercised (79,973) 7.01 — — (36,640) 7.70 Forfeited/canceled (7,726) 41.04 — — — — Outstanding at end of year 2,485,808 $ 11.60 563,230 $ 10.42 563,230 $ 10.42 Exercisable at end of year 2,172,052 $ 11.86 563,230 $ 10.42 563,230 $ 10.42
Incentive Compensation and Other Benefit Plans for Employees and Directors [Table Text Block]	For the last three years, grants of restricted and/or deferred shares have been awarded to elected or re-elected nonemployee directors as follows: Year of Grant Restricted Shares Deferred Shares 2020 253,809 54,794 2019 86,477 23,659 2018 92,718 17,170

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block]	Income (loss) from continuing operations before income taxes includes the following components: (In Millions) 2020 2019 2018 United States $ (201) $ 313 $ 565 Foreign 8 — — $ (193) $ 313 $ 565
Schedule of Components of Income Tax Expense (Benefit) [Table Text Block]	The components of the income tax provision (benefit) on continuing operations consist of the following: (In Millions) 2020 2019 2018 Current provision (benefit): United States federal $ (2) $ (1) $ (1) United States state & local — — — Foreign (1) — 1 (3) (1) — Deferred provision (benefit): United States federal (95) 19 (475) United States state & local (11) — — Foreign (2) — — Total income tax provision (benefit) from continuing operations $ (111) $ 18 $ (475) The components of income taxes for other than continuing operations consisted of the following: (In Millions) 2020 2019 2018 Other comprehensive income: Postretirement benefit liability $ (52) $ 11 $ 4 Unrealized net loss on derivative financial instruments (1) — 1 Total $ (53) $ 11 $ 5
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	Reconciliation of our income tax attributable to continuing operations computed at the U.S. federal statutory rate is as follows: (In Millions) 2020 2019 2018 Tax at U.S. statutory rate $ (41) 21 % $ 66 21 % $ 119 21 % Increase (decrease) due to: Percentage depletion in excess of cost depletion (42) 22 (49) (16) (55) (10) Non-taxable income related to noncontrolling interests (9) 4 — — — — Luxembourg legal entity reduction — — 846 271 162 29 Valuation allowance release: Current year activity — — — — (80) (14) Release of U.S. valuation allowance — — — — (461) (81) Luxembourg legal entity reduction — — (846) (271) (162) (29) State taxes, net (11) 6 — — — — Other items, net (8) 4 1 1 2 — Provision for income tax expense (benefit) and effective income tax rate including discrete items $ (111) 57 % $ 18 6 % $ (475) (84) %
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]	Significant components of our deferred tax assets and liabilities are as follows: (In Millions) 2020 2019 Deferred tax assets: Operating loss and other carryforwards $ 1,236 $ 795 Pension and OPEB liabilities 228 114 Property, plant and equipment and mineral rights — 1 State and local 132 71 Other liabilities 190 70 Total deferred tax assets before valuation allowance 1,786 1,051 Deferred tax asset valuation allowance (836) (441) Net deferred tax assets 950 610 Deferred tax liabilities: Investment in partnerships (144) (137) Property, plant and equipment and mineral rights (246) — Other assets (68) (14) Total deferred tax liabilities (458) (151) Net deferred tax assets $ 492 $ 459
Summary of Valuation Allowance [Table Text Block]	The changes in the valuation allowance are presented below: (In Millions) 2020 2019 2018 Balance at beginning of year $ 441 $ 1,287 $ 1,983 Change in valuation allowance: Included in income tax expense (benefit) (3) (846) (691) Change in deferred assets in other comprehensive income — — (5) Increase from acquisitions 398 — — Balance at end of year $ 836 $ 441 $ 1,287
Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block]	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: (In Millions) 2020 2019 2018 Unrecognized tax benefits balance as of January 1 $ 29 $ 29 $ 33 Increase for tax positions in current year 7 — 4 Decrease for tax positions of prior year (4) — — Lapses in statutes of limitations — — (8) Increases from acquisitions 75 — — Unrecognized tax benefits balance as of December 31 $ 107 $ 29 $ 29

LEASE OBLIGATIONS (Tables)

LEASE OBLIGATIONS (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Lease, Cost	Lease costs are presented below: (In Millions) Year Ended December 31, 2020 2019 Operating leases $ 43 $ 4 Finance leases: Amortization of lease cost 15 6 Interest on lease liabilities 4 2 Short-term leases 13 6 Total $ 75 $ 18 Other information related to leases was as follows: (In Millions) Year Ended December 31, 2020 2019 Cash paid for amounts included in measurement of lease liabilities: Operating leases within cash flows from operating activities $ 43 $ 4 Finance leases within cash flows from operating activities $ 4 $ 2 Finance leases within cash flows from financing activities $ 15 $ 6 Right-of-use assets obtained in exchange for new finance lease liabilities 1 $ 44 $ 29 Weighted-average remaining lease term - operating leases (in years) 8 10 Weighted-average remaining lease term - finance leases (in years) 5 6 Weighted-average discount rate - operating leases 8 % 8 % Weighted-average discount rate - finance leases 4 % 5 % 1 Right-of-use assets obtained in the Acquisitions are not included in this figure.
Lessee, Operating and Finance Lease, Liability, Maturity	Future minimum lease payments under noncancellable finance and operating leases as of December 31, 2020 were as follows: (In Millions) Finance Operating Leases 2021 $ 100 $ 70 2022 95 53 2023 78 46 2024 23 38 2025 22 34 Thereafter 76 122 Total future minimum lease payments 394 363 Less: imputed interest 59 99 Total lease payments 335 264 Less: current portion of lease liabilities 91 53 Long-term lease liabilities $ 244 $ 211

ASSET RETIREMENT OBLIGATIONS (T

ASSET RETIREMENT OBLIGATIONS (Tables)	12 Months Ended
	Dec. 31, 2020
Environmental Remediation Obligations [Abstract]
Schedule of Asset Retirement Obligations	The following is a summary of our asset retirement obligations: (In Millions) December 31, 2020 2019 Asset retirement obligations 1 $ 342 $ 165 Less: current portion 7 2 Long-term asset retirement obligations $ 335 $ 163 1 Includes $190 million and $22 million related to our active operations as of December 31, 2020 and 2019, respectively.
Asset Retirement Obligations	The following is a roll-forward of our asset retirement obligation liability: (In Millions) 2020 2019 Asset retirement obligation as of January 1 $ 165 $ 172 Increase from acquisitions 172 — Accretion expense 14 10 Remediation payments (9) (1) Revision in estimated cash flows — (16) Asset retirement obligation as of December 31 $ 342 $ 165

CAPITAL STOCK (Tables)

CAPITAL STOCK (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Dividends Declared [Table Text Block]	The below table summarizes our recent dividend activity: Declaration Date Record Date Payment Date Dividend Declared per Common Share 1 2/18/2020 4/3/2020 4/15/2020 $ 0.06 12/2/2019 1/3/2020 1/15/2020 0.06 9/3/2019 10/4/2019 10/15/2019 0.10 5/31/2019 7/5/2019 7/15/2019 0.06 2/19/2019 4/5/2019 4/15/2019 0.05 10/18/2018 1/4/2019 1/15/2019 0.05 1 The dividend declared on September 3, 2019 included a special cash dividend of $0.04 per common share.

ACCUMULATED OTHERCOMPREHENSIVE

ACCUMULATED OTHERCOMPREHENSIVE INCOME (LOSS) (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The components of Accumulated other comprehensive loss within Cliffs shareholders’ equity and related tax effects allocated to each are shown below: (In Millions) Pre-tax Tax After-tax As of December 31, 2020: Postretirement benefit liability $ (221) $ 86 $ (135) Foreign currency translation adjustments 3 — 3 Unrealized net loss on derivative financial instruments (1) — (1) $ (219) $ 86 $ (133) As of December 31, 2019: Postretirement benefit liability $ (454) $ 138 $ (316) Unrealized net loss on derivative financial instruments (4) 1 (3) $ (458) $ 139 $ (319) As of December 31, 2018: Postretirement benefit liability $ (408) $ 127 $ (281) Unrealized net loss on derivative financial instruments (4) 1 (3) $ (412) $ 128 $ (284) The following table reflects the changes in Accumulated other comprehensive loss related to Cliffs shareholders’ equity: (In Millions) Postretirement Benefit Liability, Foreign Currency Translation Derivative Financial Instruments, Accumulated Other Comprehensive Loss December 31, 2017 $ (264) $ 225 $ — $ (39) Other comprehensive income (loss) before reclassifications (43) 3 (1) (41) Net loss (gain) reclassified from accumulated other comprehensive loss 26 (228) (2) (204) December 31, 2018 (281) — (3) (284) Other comprehensive loss before reclassifications (57) — (2) (59) Net loss reclassified from accumulated other comprehensive loss 22 — 2 24 December 31, 2019 (316) — (3) (319) Other comprehensive income (loss) before reclassifications 163 3 (6) 160 Net loss reclassified from accumulated other comprehensive loss 18 — 8 26 December 31, 2020 $ (135) $ 3 $ (1) $ (133)
Reclassification out of Accumulated Other Comprehensive Income [Table Text Block]	The following table reflects the details about Accumulated other comprehensive loss components reclassified from Cliffs shareholders’ equity: (In Millions) Details about Accumulated Other Comprehensive Loss Components Amount of (Gain)/Loss Affected Line Item in the Statement of Consolidated Operations Year Ended December 31, 2020 2019 2018 Amortization of pension and OPEB liability: Prior service costs 1 $ (1) $ (1) $ (1) Other non-operating income Net actuarial loss 1 30 29 27 Other non-operating income Settlements 1 (6) — — Other non-operating income 23 28 26 Total before taxes Income tax expense (5) (6) — Income tax benefit (expense) $ 18 $ 22 $ 26 Net of taxes Changes in foreign currency translation: Gain on foreign currency translation 2 $ — $ — $ (228) Income (loss) from discontinued operations, net of tax Changes in derivative financial instruments: Commodity contracts $ 10 $ 3 $ (2) Cost of goods sold Income tax expense (2) (1) — Income tax benefit (expense) $ 8 $ 2 $ (2) Net of taxes Total reclassifications for the period, net of tax $ 26 $ 24 $ (204) 1 These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost. See NOTE 10 - PENSIONS AND OTHER POSTRETIREMENT BENEFITS for further information. 2 Represents Australian accumulated currency translation adjustments due to the liquidation of our Australian subsidiaries' net assets.

RELATED PARTIES (Tables)

RELATED PARTIES (Tables)	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Summary Of Related Party Transactions Table Disclosure	Revenues from related parties were as follows: (In Millions) Year Ended December 31, 2020 2019 2018 Revenue from related parties $ 893 $ 1,015 $ 1,344 Revenues 1 $ 5,354 $ 1,990 $ 2,332 Related party revenues as a percent of Revenues 1 16.7 % 51.0 % 57.6 % Purchases from related parties $ 16 $ — $ — 1 Includes Realization of deferred revenue of $35 million for the year ended December 31, 2020.
Summary of Balance Sheet Presentation	The following table presents the classification of related party assets and liabilities in the Statements of Consolidated Financial Position: (In Millions) December 31, Balance Sheet Location of Assets (Liabilities) 2020 2019 Accounts receivable, net $ 2 $ 31 Other current assets — 45 Accounts payable (6) — Other current liabilities — (2)

VARIABLE INTEREST ENTITIES (Tab

VARIABLE INTEREST ENTITIES (Tables)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Variable Interest Entities	The consolidated balance sheet as of December 31, 2020 includes the following amounts for SunCoke Middletown: (In Millions) December 31, Cash and cash equivalents $ 5 Inventories 21 Property, plant and equipment, net 308 Accounts payable (15) Other assets (liabilities), net (10) Noncontrolling interest (309)

EARNINGS PER SHARE (Tables)

EARNINGS PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Earnings Per Share Computation	The following table summarizes the computation of basic and diluted EPS: (In Millions, Except Per Share Amounts) Year Ended December 31, 2020 2019 2018 Income (loss) from continuing operations $ (82) $ 295 $ 1,040 Income from continuing operations attributable to noncontrolling interest (41) — — Net income (loss) from continuing operations attributable to Cliffs shareholders (123) 295 1,040 Income (loss) from discontinued operations, net of tax 1 (2) 88 Net income (loss) attributable to Cliffs shareholders $ (122) $ 293 $ 1,128 Weighted average number of shares: Basic 379 277 297 Convertible senior notes — 4 3 Employee stock plans — 3 4 Diluted 379 284 304 Earnings (loss) per common share attributable to Continuing operations $ (0.32) $ 1.07 $ 3.50 Discontinued operations — (0.01) 0.30 $ (0.32) $ 1.06 $ 3.80 Earnings (loss) per common share attributable to Continuing operations $ (0.32) $ 1.04 $ 3.42 Discontinued operations — (0.01) 0.29 $ (0.32) $ 1.03 $ 3.71
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following table summarizes the shares that have been excluded from the diluted earnings per share calculation for the year ended December 31, 2020, as they were anti-dilutive: (In Millions) 2020 Redeemable preferred shares 4 Convertible senior notes 2 Shares related to employee stock plans 1

COMMITMENT AND CONTINGENCIES (T

COMMITMENT AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Schedule of Environmental Obligations	The following is a summary of our environmental obligations: (In Millions) December 31, December 31, Environmental obligations $ 135 $ 2 Less current portion 18 — Long-term environmental obligations $ 117 $ 2

BASIS OF PRESENTATION AND SIG_4

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)employees	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Basis of Presentation and Significant Accounting Policies [Line Items]
Entity Number of Employees | employees	25,000
Foreign Currency Transaction Gain (Loss), Realized | $	$ 0	$ 0	$ 228
Asia Pacific Iron Ore [Member]
Basis of Presentation and Significant Accounting Policies [Line Items]
Foreign Currency Transaction Gain (Loss), Realized | $			$ 228
Workforce Subject to Collective Bargaining Arrangements [Member]
Basis of Presentation and Significant Accounting Policies [Line Items]
Entity Number of Employees | employees	18,500

BASIS OF PRESENTATION AND SIG_5

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Investments in Affiliates (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 09, 2020	Dec. 31, 2019
Other Noncurrent Assets [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	$ (105)
Other Noncurrent Liabilities [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures			$ (18)
Combined Metals of Chicago, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest, equity method investment	40.00%
Spartan Steel Coating, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest, equity method investment	48.00%
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$ 33
Hibbing [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest, equity method investment	85.30%	62.30%	23.00%

BASIS OF PRESENTATION AND SIG_6

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Depreciation Methods and Useful Lives (Details)	12 Months Ended
	Dec. 31, 2020
Land Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Land Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	45 years
Building [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Building [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	45 years
Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years

BASIS OF PRESENTATION AND SIG_7

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Other Intangible Assets and Liabilities (Details) - Weighted Average [Member]	12 Months Ended
	Dec. 31, 2020
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	18 years
Developed Technology Rights
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	17 years
Trademarks and Trade Names
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years

SUPPLEMENTARY FINANCIAL STATE_3

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses as of January 1	$ 0	$ 0
Increase in allowance	(5)	0
Allowance for credit losses as of December 31	$ (5)	$ 0

SUPPLEMENTARY FINANCIAL STATE_4

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Inventory (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure Text Block [Abstract]
Finished and semi-finished goods	$ 2,125	$ 114
Work-in-process	0	69
Raw materials	1,431	9
Total product inventories	3,556	192
Manufacturing supplies and critical spares	272	125
Inventories	3,828	317
Excess of Replacement or Current Costs over Stated LIFO Value	104	101
Effect of LIFO Inventory Liquidation on Income	30
Effect of LIFO Inventory Increment on Financial Position		34
Allowance for Obsolete and Surplus Items	$ 13	$ 13

SUPPLEMENTARY FINANCIAL STATE_5

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Text Block [Abstract]
Capital Additions	$ 483	$ 690	$ 395
Non-cash accruals	(86)	15	94
Right-of-use asset - finance leases	44	29	8
Grants	0	(10)	(3)
Cash paid for capital expenditures including deposits	525	656	296
Taxes paid on income	5	0	3
Income tax refunds	(120)	(118)	(11)
Interest paid on debt obligations net of capitalized interest	170	98	106
Interest capitalized	$ 53	$ 25	$ 7

SUPPLEMENTARY FINANCIAL STATE_6

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Non-cash Investing and Financing Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ArcelorMittal USA [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given, Pre-existing Relationship	$ 237	$ 0	$ 0
ArcelorMittal USA [Member] | Common Stock [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	990	0	0
ArcelorMittal USA [Member] | Series B Preferred Stock
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	738	0	0
AK Steel [Member] | Common Stock [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	618	0	0
Share-based Payment Arrangement [Member] | AK Steel [Member]
Noncash or Part Noncash Acquisitions [Line Items]
Other Significant Noncash Transaction, Value of Consideration Given	$ 4	$ 0	$ 0

SUPPLEMENTARY FINANCIAL STATE_7

SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Discontinued Operation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax	$ 1	$ (2)	$ 88
Operating income (loss)	(142)	429	673
Foreign Currency Transaction Gain (Loss), Realized	$ 0	$ 0	228
Asia Pacific Iron Ore [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating income (loss)			(105)
Foreign Currency Transaction Gain (Loss), Realized			$ 228

ACQUISITIONS - Narrative (Detai

ACQUISITIONS - Narrative (Details) $ in Millions	Dec. 09, 2020USD ($)	Mar. 13, 2020	Dec. 31, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Business Acquisition [Line Items]
Revenues				$ 5,354	$ 1,990	$ 2,332
Net income (loss) attributable to Cliffs shareholders				(122)	293	1,128
Amortization of Inventory Step-up				96	0	0
Severance Costs				38	2	0
Business Combination, Acquisition Related Costs				90	$ 7	$ 0
ArcelorMittal USA [Member]
Business Acquisition [Line Items]
Revenues	$ 446
Net income (loss) attributable to Cliffs shareholders	(40)
Amortization of Inventory Step-up	$ 21
Business Combination, Acquisition Related Costs				26
AK Steel [Member]
Business Acquisition [Line Items]
Revenues			$ 3,573
Net income (loss) attributable to Cliffs shareholders			(302)
Amortization of Inventory Step-up			74
Severance Costs			$ 35
Business Combination, Acquisition Related Costs				$ 26
Business Acquisition, Shares Exchange Ratio		0.400
AK Steel [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Business Acquisition, Shares Exchange Ratio				0.400

ACQUISITIONS - Consideration (D

ACQUISITIONS - Consideration (Details) $ / shares in Units, $ in Millions	Dec. 09, 2020USD ($)$ / sharesshares	Mar. 13, 2020USD ($)$ / sharesshares	Dec. 31, 2020shares	Dec. 31, 2019shares
Business Acquisition [Line Items]
Cash consideration (subject to customary working capital adjustments)	$ 631
Common shares, outstanding (in shares) | shares			477,517,372	270,084,005
Series B Preferred Stock
Business Acquisition [Line Items]
Fair value of Cliffs issued stock	738
ArcelorMittal USA [Member]
Business Acquisition [Line Items]
Fair value of settlement of a pre-existing relationship	237
Cash consideration (subject to customary working capital adjustments)	631
Total purchase consideration	2,596
Payments to Acquire Businesses, Pursuant to Transaction Agreement	505
Payments to Acquire Businesses, Purchase of JV Interest	182
Payments to Acquire Businesses, Cash Consideration Receivable	(56)
Business Combination, Separately Recognized Transactions, Accounts Receivable	97
Business Combination, Separately Recognized Transactions, Assets Recognized	140
ArcelorMittal USA [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Fair value of Cliffs issued stock	$ 990
Number of shares issued (in shares) | shares	78,186,671		78,000,000
Share price (in dollars per share) | $ / shares	$ 12.66
ArcelorMittal USA [Member] | Series B Preferred Stock
Business Acquisition [Line Items]
Number of shares issued (in shares) | shares	583,273		583,273
Share price (in dollars per share) | $ / shares	$ 1,266
AK Steel [Member]
Business Acquisition [Line Items]
Total purchase consideration		$ 1,535
Share price (in dollars per share) | $ / shares		$ 4.87
Business Combination, Consideration Transferred, Liabilities Incurred		$ 914
Common shares, outstanding (in shares) | shares		317,000,000
Business Acquisition, Shares Exchange Ratio		0.400
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares		127,000,000
AK Steel [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Number of shares issued (in shares) | shares			127,000,000
Business Acquisition, Shares Exchange Ratio			0.400
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned		$ 618
Business Combination, Consideration Transferred, Other		3
AK Steel [Member] | ABL Facility [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Liabilities Incurred		590
AK Steel [Member] | 7.50% 2023 AK Senior Notes [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred, Liabilities Incurred		$ 324

ACQUISITIONS - Preliminary purc

ACQUISITIONS - Preliminary purchase price allocation (Details) - USD ($) $ in Millions	10 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Mar. 13, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Assets, Income Tax Receivable	$ 12	$ 12	$ 3
Goodwill	1,406	1,406		$ 2
ArcelorMittal USA [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	35	35
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables	349	349
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory	2,115	2,115
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	22	22
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	4,017	4,017
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	158	158
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	(758)	(758)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Employment Costs	(271)	(271)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, State and Local Taxes	(76)	(76)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Pension and OPEB Liabilities	(109)	(109)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(322)	(322)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Pension and OPEB Liabilities	(3,195)	(3,195)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	(598)	(598)
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	(13)	(13)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	1,366	1,366
Goodwill	1,230	1,230
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest	2,596	2,596
AK Steel [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	39	39	38
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables	664	664	666
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory	1,320	1,320	1,563
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Assets, Income Tax Receivable	3	3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other	49	49	65
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	2,312	2,312	2,184
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Income Taxes	30	30	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	472	472	475
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	(644)	(644)	(636)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Employment Costs	(93)	(93)	(94)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, State and Local Taxes	(31)	(31)	(35)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Pension and OPEB Liabilities	(78)	(78)	(75)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(196)	(196)	(201)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	1,179	1,179	1,179
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Pension and OPEB Liabilities	(871)	(871)	(873)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	(435)	(435)	(507)
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	(1)	(1)	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	1,361	1,361	1,394
Goodwill	174	174	141
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest	1,535	1,535	$ 1,535
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Cash and Cash Equivalents	1
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable	(2)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory	(243)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Income Tax Receivables	0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets	(16)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment	128
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes	30
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Noncurrent Assets	(3)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable	(8)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Employment Cost	1
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, State and Local Income Taxes	4
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Retirement Benefit Liabilities	(3)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Liabilities	5
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Long-term Debt	0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Retirement Benefit Liabilities, Noncurrent	2
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Noncurrent Liabilities	72
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Noncontrlling Interest	(1)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Identifiable Assets	(33)
Goodwill, Purchase Accounting Adjustments	33
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred	$ 0
AK Steel [Member] | Inventories [Member]
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory		243
AK Steel [Member] | Cost of goods sold
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments Related to Previous Period		$ 8

ACQUISITIONS - Intangible asset

ACQUISITIONS - Intangible assets and liabilities (Details) - AK Steel [Member] $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 148
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	17 years
Finite-lived Intangible Liability Assumed	$ (71)
Assumed Finite-lived Intangible Liabilities, Weighted Average Useful Life	12 years
Customer Relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 77
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	18 years
Technology-Based Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 60
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	17 years
Trademarks and Trade Names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 11
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years

ACQUISITIONS - Pro Forma (Detai

ACQUISITIONS - Pro Forma (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Acquisition, Pro Forma Revenue	$ 12,837	$ 17,163
Business Acquisition, Pro Forma Net Income (Loss)	(520)	(11)
Business Acquisition, Pro Forma Expenses	$ 32	76
Business Acquisition, Pro Forma Income Tax, Statutory Rate	24.30%
Business Acquisition, Pro Forma Income Tax Benefit (Expense)	$ 170	117
Intersegment Eliminations [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Acquisition, Pro Forma Revenue	844	1,499
Franchise
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Acquisition, Pro Forma Net Income (Loss)	140	129
Fair Value Adjustment to Inventory
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Acquisition, Pro Forma Net Income (Loss)	96	362
Acquisition-related Costs | ArcelorMittal USA and AK Steel [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Acquisition, Pro Forma Net Income (Loss)	93	$ 93
Restructuring Costs | ArcelorMittal USA [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Business Acquisition, Pro Forma Net Income (Loss)	$ 1,820

REVENUES - By Market and Produc

REVENUES - By Market and Product Line (Details) T in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)T	Dec. 31, 2019USD ($)T	Dec. 31, 2018USD ($)T
Disaggregation of Revenue [Line Items]
Revenues	$ 5,354	$ 1,990	$ 2,332
Realization of Deferred Revenue	35	0	0
Customer Supply Agreements [Member] | Sales [Member]
Disaggregation of Revenue [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	122	78	426
Steelmaking [Member]
Disaggregation of Revenue [Line Items]
Revenues	4,965	1,990	2,332
Steelmaking [Member] | Hot-rolled steel
Disaggregation of Revenue [Line Items]
Revenues	$ 386	$ 0	$ 0
Sales Volume (Net Tons) | T	633	0	0
Steelmaking [Member] | Cold-rolled steel
Disaggregation of Revenue [Line Items]
Revenues	$ 490	$ 0	$ 0
Sales Volume (Net Tons) | T	682	0	0
Steelmaking [Member] | Coated steel
Disaggregation of Revenue [Line Items]
Revenues	$ 1,747	$ 0	$ 0
Sales Volume (Net Tons) | T	1,911	0	0
Steelmaking [Member] | Stainless and electrical steel
Disaggregation of Revenue [Line Items]
Revenues	$ 868	$ 0	$ 0
Sales Volume (Net Tons) | T	416	0	0
Steelmaking [Member] | Other steel products
Disaggregation of Revenue [Line Items]
Revenues	$ 92	$ 0	$ 0
Sales Volume (Net Tons) | T	141	0	0
Steelmaking [Member] | Iron products
Disaggregation of Revenue [Line Items]
Revenues	$ 1,335	$ 1,990	$ 2,332
Sales Volume (Long Tons) | T	11,707	18,583	20,563
Steelmaking [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 47	$ 0	$ 0
Other businesses
Disaggregation of Revenue [Line Items]
Revenues	389	0	0
Other businesses | Other [Member]
Disaggregation of Revenue [Line Items]
Revenues	389	0	0
Automotive [Member] | Steelmaking [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,062	0	0
Automotive [Member] | Other businesses
Disaggregation of Revenue [Line Items]
Revenues	329	0	0
Infrastructure and manufacturing | Steelmaking [Member]
Disaggregation of Revenue [Line Items]
Revenues	784	0	0
Infrastructure and manufacturing | Other businesses
Disaggregation of Revenue [Line Items]
Revenues	34	0	0
Distributors and converters | Steelmaking [Member]
Disaggregation of Revenue [Line Items]
Revenues	696	0	0
Distributors and converters | Other businesses
Disaggregation of Revenue [Line Items]
Revenues	26	0	0
Steel producers | Steelmaking [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 1,423	$ 1,990	$ 2,332

REVENUES - Deferred Revenue (De

REVENUES - Deferred Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Revenue Arrangement [Line Items]
Realization of Deferred Revenue	$ 35	$ 0	$ 0
Other Current Liabilities [Member]
Deferred Revenue Arrangement [Line Items]
Deferred Revenue (Current)	7	22	21
Net Increase (Decrease)	(15)	1
Other Noncurrent Liabilities [Member]
Deferred Revenue Arrangement [Line Items]
Net Increase (Decrease)	(26)	(13)
Deferred Revenue (Long-Term)	$ 0	$ 26	$ 39

SEGMENT REPORTING - Schedule Of

SEGMENT REPORTING - Schedule Of Segment Reporting Information, By Segment (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)segments	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments | segments	4
Number of Reportable Segments | segments	1
Revenues	$ 5,354	$ 1,990	$ 2,332
Adjusted EBITDA, Earnings (Loss)	353	525	766
Realization of Deferred Revenue	35	0	0
Steelmaking [Member]
Segment Reporting Information [Line Items]
Revenues	4,965	1,990	2,332
Adjusted EBITDA, Earnings (Loss)	433	636	872
Other businesses
Segment Reporting Information [Line Items]
Revenues	389	0	0
Adjusted EBITDA, Earnings (Loss)	47	0	0
Corporate and Other Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA, Earnings (Loss)	$ (127)	$ (111)	$ (106)

SEGMENT REPORTING - Reconciliat

SEGMENT REPORTING - Reconciliation of Net Income (Loss) to Adjusted EBITDA (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting [Abstract]
Net income (loss)	$ (81)	$ 293	$ 1,128
Interest expense, net	(238)	(101)	(121)
Income tax benefit (expense)	111	(18)	460
Depreciation, depletion and amortization	(308)	(85)	(89)
EBITDA	354	497	878
EBITDA of Noncontrolling Interests	56	0	0
Gain (loss) on extinguishment of debt	130	(18)	(7)
Severance Costs	(38)	(2)	0
Acquisition-related costs excluding severance costs	(52)	(7)	0
Amortization of Inventory Step-up	(96)	0	0
EBITDA Earnings (loss) from Discontinued Operations	(1)	1	(121)
Foreign exchange remeasurement	0	0	(1)
Asset Impairment Charges	0	0	(1)
Adjusted EBITDA, Earnings (Loss)	353	525	766
Net Income (Loss) Attributable to Noncontrolling Interest	(41)	$ 0	$ 0
Depreciation, Depletion and Amortization Attributable to Noncontrolling Interests	$ 15

SEGMENT REPORTING - Depreciatio

SEGMENT REPORTING - Depreciation, Depletion and Amortization and Capital Additions By Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	$ 308	$ 85	$ 89
Depreciation, Depletion and Amortization excluding Depreciation, Amortization and Depletion expense for Discontinued Operations			74
Property, Plant and Equipment, Additions	483	690	395
Steelmaking [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	276	80	68
Property, Plant and Equipment, Additions	436	687	393
Other businesses
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	27	0	0
Property, Plant and Equipment, Additions	45	0	0
Corporate and Other Segments [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	5	5	6
Property, Plant and Equipment, Additions	$ 2	$ 3	$ 2

SEGMENT REPORTING - Summary of

SEGMENT REPORTING - Summary of Assets by Segment (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 16,771	$ 3,504
Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	16,088	2,557
Steelmaking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	15,849	2,557
Other businesses | Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	239	0
Corporate and Other Segments [Member]
Segment Reporting Information [Line Items]
Assets	$ 683	$ 947

SEGMENTS REPORTING (Revenues an

SEGMENTS REPORTING (Revenues and Property, Plant and Equipment, Net by Geographical Location) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 5,354	$ 1,990	$ 2,332
Property, plant and equipment, net	8,743	1,929	1,286
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	4,580	1,505	1,847
Property, plant and equipment, net	8,647	1,929	1,286
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	602	448	395
Property, plant and equipment, net	91	0	0
Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	172	37	90
Property, plant and equipment, net	$ 5	$ 0	$ 0

PROPERTY, PLANT AND EQUIPMENT -

PROPERTY, PLANT AND EQUIPMENT - Value Of Each Of The Major Classes Of Consolidated Depreciable Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 10,217	$ 3,027
Allowance for depreciation and depletion	(1,474)	(1,098)
Property, plant and equipment, net	8,743	1,929	$ 1,286
Finance Lease, Right-of-Use Asset, after Accumulated Amortization	361	49
Land, land improvements and mineral rights [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,213	582
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	703	158
Machinery and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	6,786	1,456
Other Capitalized Property Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	151	101
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 1,364	$ 730

PROPERTY, PLANT AND EQUIPMENT_2

PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation	$ 298	$ 77	$ 66
Interest capitalized	53	25	7
Depletion	$ 8	$ 8	$ 7

PROPERTY, PLANT AND EQUIPMENT_3

PROPERTY, PLANT AND EQUIPMENT (Book Value of Land and Mineral Rights Disclosure) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Mineral rights:
Mineral Properties, Gross	$ 773	$ 537
Mineral Properties, Accumulated Depletion	(142)	(134)
Mineral Properties, Net	631	403
Land Rights	$ 361	$ 12

GOODWILL AND INTANGIBLE ASSET_3

GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES - Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 13, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill	$ 1,406		$ 2
ArcelorMittal USA [Member]
Goodwill [Line Items]
Goodwill	1,230
AK Steel [Member]
Goodwill [Line Items]
Goodwill	174	$ 141
Steelmaking [Member]
Goodwill [Line Items]
Goodwill	1,232		2
Steelmaking [Member] | ArcelorMittal USA [Member]
Goodwill [Line Items]
Goodwill, Period Increase (Decrease)	1,230
Other businesses
Goodwill [Line Items]
Goodwill	174		$ 0
Other businesses | AK Steel [Member]
Goodwill [Line Items]
Goodwill, Period Increase (Decrease)	$ 174

GOODWILL AND INTANGIBLE ASSET_4

GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES - Intangibles (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 220
Finite-Lived Intangible Assets, Accumulated Amortization	(32)
Finite-Lived Intangible Assets, Net	188
Finite-Lived Intangible Liability, Gross	(71)
Finite-Lived Intangible Liability, Accumulated Amortization	7
Finite-Lived Intangible Liability, Net	(64)
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	77
Finite-Lived Intangible Assets, Accumulated Amortization	(3)
Finite-Lived Intangible Assets, Net	74
Technology-Based Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	60
Finite-Lived Intangible Assets, Accumulated Amortization	(3)
Finite-Lived Intangible Assets, Net	57
Trade Names
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	11
Finite-Lived Intangible Assets, Accumulated Amortization	(1)
Finite-Lived Intangible Assets, Net	10
Mining Permits [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	72	$ 72
Finite-Lived Intangible Assets, Accumulated Amortization	(25)	(24)
Finite-Lived Intangible Assets, Net	$ 47	$ 48

GOODWILL AND INTANGIBLE ASSET_5

GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of Intangible Assets	$ 8	$ 1
Finite-Lived Intangible Asset, Expected Amortization, Year One	10
Finite-Lived Intangible Asset, Expected Amortization, Year Two	10
Finite-Lived Intangible Asset, Expected Amortization, Year Three	10
Finite-Lived Intangible Asset, Expected Amortization, Year Four	10
Finite-Lived Intangible Asset, Expected Amortization, Year Five	10
Amortization of Intangible Liability	(7)
Finite-Lived Intangible Liabilities, Amortization Credit, Next Twelve Months	(8)
Finite-Lived Intangible Liabilities, Amortization Credit, Year Two	(8)
Finite-Lived Intangible Liabilities, Amortization Credit, Year Three	(8)
Finite-Lived Intangible Liabilities, Amortization Credit, Year Four	(8)
Finite-Lived Intangible Liabilities, Amortization Credit, Year Five	$ (8)

DEBT AND CREDIT FACILITIES - Sc

DEBT AND CREDIT FACILITIES - Schedule Of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jun. 19, 2020	Apr. 24, 2020	Apr. 17, 2020	Mar. 16, 2020	Mar. 13, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 5,595
Total Debt	5,424
Long-term Debt, Current Maturities	34
Long-term debt	$ 5,390						$ 2,114
Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	3.25%
Cleveland-Cliffs Inc. [Member] | ABL Facility [Member]
Debt Instrument [Line Items]
Imputed interest rate	2.15%
Debt Instrument, Par Value							450
Line of Credit Facility, Maximum Borrowing Capacity	$ 3,500
Credit facility, amount outstanding	1,510						$ 0
Cleveland-Cliffs Inc. [Member] | EDC Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	40
Credit facility, amount outstanding	18
AK Steel [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	132
AK Steel [Member] | Industrial Revenue Bonds [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	92
Unamortized Debt Issuance Expense	0
Total Debt	94
Debt Instrument, Unamortized Premium	$ 2
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	5.00%						5.00%
Debt Instrument, Par Value	$ 395						$ 400
Unamortized Debt Issuance Expense	(3)						(5)
Debt Instrument, Unamortized Discount	1						2
Total Debt	$ 391						$ 393
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	10.57%
Debt Instrument, Par Value	$ 955		$ 555	$ 400
Unamortized Debt Issuance Expense	(8)
Debt Instrument, Unamortized Discount	25
Total Debt	$ 922
6.75% 2026 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	6.99%
Debt Instrument, Par Value	$ 845	$ 120				$ 725
Unamortized Debt Issuance Expense	(20)
Debt Instrument, Unamortized Discount	9
Total Debt	$ 816
7.625% 2021 AK Senior Notes | AK Steel [Member]
Debt Instrument [Line Items]
Imputed interest rate	7.33%
Debt Instrument, Par Value	$ 34
Unamortized Debt Issuance Expense	0
Debt Instrument, Unamortized Discount	0
Total Debt	$ 34
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member]
Debt Instrument [Line Items]
Imputed interest rate	6.17%
Debt Instrument, Par Value	$ 13
Unamortized Debt Issuance Expense	0
Debt Instrument, Unamortized Discount	0
Total Debt	$ 13
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	8.11%
Debt Instrument, Par Value	$ 64				$ 232
Unamortized Debt Issuance Expense	0
Debt Instrument, Unamortized Discount	4
Total Debt	$ 60
6.375% 2025 AK Senior Notes | AK Steel [Member]
Debt Instrument [Line Items]
Imputed interest rate	8.11%
Debt Instrument, Par Value	$ 29
Unamortized Debt Issuance Expense	0
Debt Instrument, Unamortized Discount	2
Total Debt	$ 27
1.50% 2025 Convertible Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	6.26%						6.26%
Debt Instrument, Par Value	$ 296						$ 316
Unamortized Debt Issuance Expense	(4)						(4)
Debt Instrument, Unamortized Discount	49						65
Total Debt	$ 243						$ 247
5.75% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	6.01%						6.01%
Debt Instrument, Par Value	$ 396						$ 473
Unamortized Debt Issuance Expense	(3)						(3)
Debt Instrument, Unamortized Discount	4						6
Total Debt	$ 389						$ 464
7.00% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	9.24%
Debt Instrument, Par Value	$ 73				$ 335
Unamortized Debt Issuance Expense	0
Debt Instrument, Unamortized Discount	8
Total Debt	$ 65
7.00% 2027 AK Senior Notes | AK Steel [Member]
Debt Instrument [Line Items]
Imputed interest rate	9.24%
Debt Instrument, Par Value	$ 56
Unamortized Debt Issuance Expense	0
Debt Instrument, Unamortized Discount	6
Total Debt	$ 50
5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	6.49%						6.49%
Debt Instrument, Par Value	$ 556						$ 750
Unamortized Debt Issuance Expense	(4)						(6)
Debt Instrument, Unamortized Discount	18						27
Total Debt	$ 534						$ 717
6.25% 2040 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Imputed interest rate	6.34%						6.34%
Debt Instrument, Par Value	$ 263						$ 298
Unamortized Debt Issuance Expense	(2)						(2)
Debt Instrument, Unamortized Discount	3						3
Total Debt	$ 258						$ 293

DEBT AND CREDIT FACILITIES - De

DEBT AND CREDIT FACILITIES - Debt Issuances and Terms (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)$ / shares	Dec. 31, 2018	Jun. 19, 2020USD ($)	Apr. 24, 2020USD ($)	Apr. 17, 2020USD ($)	Mar. 16, 2020USD ($)	Mar. 13, 2020USD ($)	Dec. 31, 2019USD ($)
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 5,595
Common shares, par value | $ / shares	$ 0.125
AK Steel [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 132
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 955			$ 555	$ 400
Debt issuance, discount rate				99.00%	94.50%
Repurchase price if triggering event occurs	101.00%
6.75% 2026 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 845		$ 120				$ 725
Debt issuance, discount rate			99.25%				98.783%
Repurchase price if triggering event occurs	101.00%
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 64					$ 232
Repurchase price if triggering event occurs	101.00%
7.00% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 73					$ 335
Repurchase price if triggering event occurs	101.00%
1.50% 2025 Convertible Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Convertible, Conversion Ratio	129.2985	122.4365
Convertible debt, initial conversion price | $ / shares	$ 8.17
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger, Five-Day Trading	0.98
Convertible debt, equity component	$ 86
Debt Instrument, Redemption Price, Percentage	100.00%
1.50% 2025 Convertible Senior Notes | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger	130.00%
1.50% 2025 Convertible Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 296							$ 316
5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 556							750
Repurchase price if triggering event occurs	101.00%
6.25% 2040 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 263							298
Repurchase price if triggering event occurs	101.00%
5.75% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 396							473
Repurchase price if triggering event occurs	101.00%
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 395							$ 400
Repurchase price if triggering event occurs	101.00%

DEBT AND CREDIT FACILITIES - _2

DEBT AND CREDIT FACILITIES - Debt Redemption (Details)	12 Months Ended
	Dec. 31, 2020
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	109.875%
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period Two | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	107.406%
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period Three | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	104.938%
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period Four | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	102.469%
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period Five [Member] | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	106.75%
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period Two | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	105.063%
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period Three | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	103.375%
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period Four | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.688%
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period Five [Member] | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
4.875% 2024 Senior Secured Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	102.438%
4.875% 2024 Senior Secured Notes | Debt Instrument, Redemption, Period Two | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.219%
4.875% 2024 Senior Secured Notes | Debt Instrument, Redemption, Period Three | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
6.375% 2025 Senior Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	103.188%
6.375% 2025 Senior Notes | Debt Instrument, Redemption, Period Two | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.594%
6.375% 2025 Senior Notes | Debt Instrument, Redemption, Period Three | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
7.00% 2027 Senior Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
7.00% 2027 Senior Notes | Debt Instrument, Redemption, Period Two | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	103.50%
7.00% 2027 Senior Notes | Debt Instrument, Redemption, Period Three | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	102.333%
7.00% 2027 Senior Notes | Debt Instrument, Redemption, Period Four | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.167%
7.00% 2027 Senior Notes | Debt Instrument, Redemption, Period Five [Member] | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
7.50% 2023 AK Senior Notes [Member] | Debt Instrument, Redemption, Period One | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.875%
7.50% 2023 AK Senior Notes [Member] | Debt Instrument, Redemption, Period Two | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
6.375% 2025 AK Senior Notes | Debt Instrument, Redemption, Period One | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	103.188%
6.375% 2025 AK Senior Notes | Debt Instrument, Redemption, Period Two | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.594%
6.375% 2025 AK Senior Notes | Debt Instrument, Redemption, Period Three | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
7.00% 2027 AK Senior Notes | Debt Instrument, Redemption, Period One | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
7.00% 2027 AK Senior Notes | Debt Instrument, Redemption, Period Two | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	103.50%
7.00% 2027 AK Senior Notes | Debt Instrument, Redemption, Period Three | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	102.333%
7.00% 2027 AK Senior Notes | Debt Instrument, Redemption, Period Four | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	101.167%
7.00% 2027 AK Senior Notes | Debt Instrument, Redemption, Period Five [Member] | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
1.50% 2025 Convertible Senior Notes
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
5.875% 2027 Senior Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	105.875%
5.875% 2027 Senior Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
6.25% 2040 Senior Notes | Debt Instrument, Redemption, Period One | Cleveland-Cliffs Inc. [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
7.625% 2021 AK Senior Notes | Debt Instrument, Redemption, Period Two | AK Steel [Member]
Debt Instrument, Redemption [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%

DEBT AND CREDIT FACILITIES - AB

DEBT AND CREDIT FACILITIES - ABL Facility (Details) - USD ($)	Dec. 31, 2020	Dec. 09, 2020	Mar. 13, 2020
ABL Facility [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 3,350,000,000	$ 2,000,000,000
Credit Facility, Sublimit For Swingline Loans			125,000,000
Filo Tranche Commitments		150,000,000	150,000,000
Line of Credit Facility, Maximum Borrowing Capacity, Tranche A		$ 1,500,000,000
Letter of Credit [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 555,000,000
ABL Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 3,500,000,000
Borrowings	1,510,000,000
Line of Credit Facility, Remaining Borrowing Capacity	1,743,000,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, amount outstanding	$ (247,000,000)

DEBT AND CREDIT FACILITIES - Ot

DEBT AND CREDIT FACILITIES - Other Financing Arrangements (Details) $ in Millions	Dec. 31, 2020USD ($)
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 5,595
AK Steel [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	132
Cleveland-Cliffs Inc. [Member] | EDC Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	40
Credit facility, amount outstanding	18
Unsecured Industrial Revenue Bonds | AK Steel [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 66
Debt, Weighted Average Interest Rate	6.86%
Secured Industrial Revenue Bonds | AK Steel [Member]
Debt Instrument [Line Items]
Debt Instrument, Par Value	$ 26

DEBT AND CREDIT FACILITIES - _3

DEBT AND CREDIT FACILITIES - Debt Extinguishments (Details) - USD ($) $ in Millions	Mar. 12, 2021	Mar. 11, 2021	Apr. 24, 2020	Mar. 27, 2020	Mar. 13, 2020	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 17, 2020	Mar. 16, 2020
Extinguishment of Debt [Line Items]
Debt Extinguished			$ 736				$ 1,513	$ 724	$ 227
Gain (loss) on extinguishment of debt							130	(18)	(7)
Debt Instrument, Par Value							5,595
Reduction of Principal Long-Term Debt			181
AK Steel [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							132
Senior Notes [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished							35
Unsecured Industrial Revenue Bonds | AK Steel [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished							7
7.625% 2021 AK Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished				$ 9	$ 364		373
Gain (loss) on extinguishment of debt							0
7.625% 2021 AK Senior Notes | AK Steel [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							34
7.625% 2021 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished	$ 34
7.50% 2023 AK Senior Notes [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished				$ 56	$ 311		367
Gain (loss) on extinguishment of debt							3
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							13
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished	13
4.875% 2024 Senior Secured Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							6
Gain (loss) on extinguishment of debt							1
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							395	400
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished	395
6.375% 2025 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							168
Gain (loss) on extinguishment of debt							21
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							64				$ 232
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished	64
1.50% 2025 Convertible Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							20
Gain (loss) on extinguishment of debt							1
1.50% 2025 Convertible Senior Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							296	316
5.75% 2025 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							77	600	2
Gain (loss) on extinguishment of debt							16	(13)	0
5.75% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							396	473
7.00% 2027 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							262
Gain (loss) on extinguishment of debt							27
7.00% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							73				$ 335
5.875% 2027 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							195
Gain (loss) on extinguishment of debt							49
5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							556	750
6.25% 2040 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							36
Gain (loss) on extinguishment of debt							13
6.25% 2040 Senior Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							263	298
6.375% 2025 AK Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished							9
Gain (loss) on extinguishment of debt							(1)
6.375% 2025 AK Senior Notes | AK Steel [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							29
6.375% 2025 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished	$ 29
4.875% 2021 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished								124	14
Gain (loss) on extinguishment of debt								$ (5)	0
5.90% 2020 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished									89
Gain (loss) on extinguishment of debt									(3)
4.80% 2020 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished									122
Gain (loss) on extinguishment of debt									$ (4)
9.875% 2025 Senior Secured Notes | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 334				$ 334
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value			$ 555				955			$ 400
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 322
Unsecured Industrial Revenue Bonds | AK Steel [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Par Value							$ 66

DEBT AND CREDIT FACILITIES (Sch

DEBT AND CREDIT FACILITIES (Schedule of Debt Maturities) (Details) $ in Millions	Dec. 31, 2020USD ($)
Debt Disclosure [Abstract]
2021	$ 34
2022	0
2023	13
2024	457
2025	1,740
Thereafter	3,351
Long-term Debt	$ 5,595

FAIR VALUE OF FINANCIAL INSTR_3

FAIR VALUE OF FINANCIAL INSTRUMENTS - Fair Value Of Assets And Liabilities (Details) $ in Millions	Dec. 31, 2019USD ($)
Assets:
Cash equivalents	$ 188
Total Asset	233
Other Current Assets [Member] | Customer Supply Agreements [Member]
Assets:
Derivative Asset	45
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	0
Total Asset	0
Fair Value, Inputs, Level 1 [Member] | Other Current Assets [Member] | Customer Supply Agreements [Member]
Assets:
Derivative Asset	0
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash equivalents	188
Total Asset	188
Fair Value, Inputs, Level 2 [Member] | Other Current Assets [Member] | Customer Supply Agreements [Member]
Assets:
Derivative Asset	0
Fair Value, Inputs, Level 3 [Member]
Assets:
Cash equivalents	0
Total Asset	45
Fair Value, Inputs, Level 3 [Member] | Other Current Assets [Member] | Customer Supply Agreements [Member]
Assets:
Derivative Asset	$ 45

FAIR VALUE OF FINANCIAL INSTR_4

FAIR VALUE OF FINANCIAL INSTRUMENTS - Fair Value, Assets and Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured On Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance - January 1	$ 45	$ 89
Included in earnings	122	79
Settlements	(27)	(123)
Settlement of pre-existing relationship	(140)	0
Ending balance - December 31	0	45
Total gains for the period included in earnings attributable to the change in unrealized gains on assets still held at the reporting date	$ 0	$ 45

FAIR VALUE OF FINANCIAL INSTR_5

FAIR VALUE OF FINANCIAL INSTRUMENTS - Carrying Value And Fair Value Of Financial Instruments Disclosure (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 5,424	$ 2,114
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	6,065	2,237
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	3,802	2,114
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	4,446	2,237
Industrial Revenue Bonds [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	94	0
Industrial Revenue Bonds [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	91	0
Line of Credit [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | ABL Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	1,510	0
Line of Credit [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | EDC Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	18	0
Line of Credit [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | ABL Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	1,510	0
Line of Credit [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | EDC Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 18	$ 0

PENSION AND OTHER POSTRETIREMEN

PENSION AND OTHER POSTRETIREMENT BENEFITS - Obligations and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Non-current liabilities	$ (4,113)	$ (312)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Defined Benefit Plan, Accumulated Benefit Obligation	6,537	1,010
ArcelorMittal USA [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Acquired through business combinations	3,294
AK Steel [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Acquired through business combinations	949
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligations - beginning of year	1,021	906
Service cost (excluding expense)	23	17	$ 19
Interest cost	64	35	30
Actuarial (gain) loss	162	112
Benefits paid	(146)	(62)
Participant contributions	0	0
Acquired through business combinations	5,535	0
Effect of settlement	(94)	0
Other	0	13
Benefit obligations - end of year	6,565	1,021	906
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets — beginning of year	749	687
Actual return on plan assets	472	98
Participant contributions	0	0
Employer contributions	50	16
Benefits paid	(146)	(62)
Acquired through business combinations	4,301	0
Effect of settlement	(94)	0
Other	0	10
Fair value of plan assets — end of year	5,332	749	687
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Assets for Plan Benefits, Defined Benefit Plan	3	0
Current liabilities	(12)	0
Non-current liabilities	(1,224)	(272)
Total amount recognized	(1,233)	(272)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial loss	164	382
Prior service cost (credit)	6	7
Net amount recognized	170	389
Funded (Unfunded) Status	(1,233)	(272)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligations - beginning of year	255	242
Service cost (excluding expense)	8	2	2
Interest cost	19	10	8
Actuarial (gain) loss	14	19
Benefits paid	(89)	(26)
Participant contributions	22	6
Acquired through business combinations	3,528	0
Effect of settlement	0	0
Other	0	2
Benefit obligations - end of year	3,757	255	242
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets — beginning of year	260	240
Actual return on plan assets	45	35
Participant contributions	17	1
Employer contributions	30	3
Benefits paid	(88)	(19)
Acquired through business combinations	519	0
Effect of settlement	0	0
Other	0	0
Fair value of plan assets — end of year	783	260	$ 240
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Assets for Plan Benefits, Defined Benefit Plan	54	49
Current liabilities	(139)	(4)
Non-current liabilities	(2,889)	(40)
Total amount recognized	(2,974)	5
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial loss	56	73
Prior service cost (credit)	(6)	(8)
Net amount recognized	50	65
Funded (Unfunded) Status	$ (2,974)	$ 5

PENSIONS AND OTHER POSTRETIRE_3

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Components Of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Expected Amounts [Abstract]
Estimated net periodic benefit cost	$ (82)
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	23	$ 17	$ 19
Interest cost	64	35	30
Expected return on plan assets	(140)	(55)	(60)
Amortization:
Net actuarial loss	27	24	21
Amortization of prior service cost (credit)	1	1	2
Settlements	(6)	0	1
Net periodic benefit cost (credit)	(31)	22	13
Defined Benefit Plan, Expected Amounts [Abstract]
Estimated net periodic benefit cost	(168)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	8	2	2
Interest cost	19	10	8
Expected return on plan assets	(20)	(17)	(18)
Amortization:
Net actuarial loss	3	5	5
Amortization of prior service cost (credit)	(2)	(2)	(3)
Settlements	0	0	0
Net periodic benefit cost (credit)	8	$ (2)	$ (6)
Defined Benefit Plan, Expected Amounts [Abstract]
Estimated net periodic benefit cost	$ 86

PENSION AND POST RETIREMENT BEN

PENSION AND POST RETIREMENT BENEFITS (Components of Accumulated Other Comprehensive Loss (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	$ (219)	$ 50
Actual returns on assets (over) under estimate	(332)	(44)
Amortization of net actuarial loss	(27)	(24)
Amortization of prior service credit (cost)	(1)	(1)
Current year prior service cost	6	0
Other	(27)	7
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(15)	(6)
Actual returns on assets (over) under estimate	(26)	(18)
Amortization of net actuarial loss	(3)	(5)
Amortization of prior service credit (cost)	2	2
Current year prior service cost	0	0
Other	(2)	(5)
Change in Assumptions for Defined Benefit Plans, Discount Rates [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	181	106
Change in Assumptions for Defined Benefit Plans, Discount Rates [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	44	26
Change in Assumptions for Defined Benefit Plans, Demographic [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(3)	12
Change in Assumptions for Defined Benefit Plans, Demographic [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(11)	4
Changes in Assumptions for Defined Benefit Plans, Mortality [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(16)	(6)
Changes in Assumptions for Defined Benefit Plans, Mortality [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(4)	(4)
Changes in Assumptions for Defined Benefit Plans, Per Capita Claims [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	0	0
Changes in Assumptions for Defined Benefit Plans, Per Capita Claims [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(10)	(9)
Changes in Assumptions for Defined Benefit Plans, Other Actuarial (Gain) Loss [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	0	0
Changes in Assumptions for Defined Benefit Plans, Other Actuarial (Gain) Loss [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	(5)	3
Changes in Assumptions for Defined Benefit Plans, Actuarial (Gain) Loss [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	162	112
Changes in Assumptions for Defined Benefit Plans, Actuarial (Gain) Loss [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, (Gain) Loss Arising During Period, before Tax	$ 14	$ 20

PENSIONS AND OTHER POSTRETIRE_4

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Contributions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	$ 50	$ 16
Expected employer contributions, next fiscal year	202
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	30	3
Expected employer contributions, next fiscal year	128
Defined Benefit Plan, Plan Assets, Contributions by Employer	25	4
Other Postretirement Benefits Plan [Member] | Veba Trust [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions, next fiscal year	(16)
Defined Benefit Plan, Plan Assets, Contributions by Employer	0	0
Other Postretirement Benefits Plan [Member] | Direct Payments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions, next fiscal year	144
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 25	$ 4

PENSION AND OTHER POSTRETIREM_2

PENSION AND OTHER POSTRETIREMENT BENEFITS - Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2020USD ($)
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2021	$ 486
2022	462
2023	480
2024	455
2025	433
2026-2030	1,983
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2021	191
2022	185
2023	180
2024	178
2025	176
2026-2030	$ 884

PENSIONS AND OTHER POSTRETIRE_5

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Weighted-Average Assumptions Used to Determine Benefit Obligations (Details)	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.34%	3.27%
Interest crediting rate	5.25%	6.00%
Rate of compensation increase	2.56%	2.53%
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.71%	3.28%
Rate of compensation increase	3.00%	3.00%

PENSIONS AND OTHER POSTRETIRE_6

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Weighted-Average Assumptions Used to Determine Net Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Obligation discount rate	3.02%	4.27%	3.58%
Service cost discount rate	3.34%	4.35%	3.64%
Interest cost discount rate	2.53%	3.92%	3.16%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Weighted-Average Interest Crediting Rate	5.50%	6.00%	6.00%
Interest crediting rate	5.25%	6.00%
Expected return on plan assets	7.69%	8.25%	8.25%
Rate of compensation increase	2.56%	2.53%	2.49%
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Obligation discount rate	3.28%	4.29%	3.60%
Service cost discount rate	3.35%	4.49%	3.73%
Interest cost discount rate	2.51%	3.94%	3.11%
Expected return on plan assets	6.82%	7.00%	7.00%
Rate of compensation increase	3.00%	3.00%	3.00%

PENSIONS AND OTHER POSTRETIRE_7

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Assumed Health Care Cost Trend Rates (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Postemployment Benefits [Abstract]
Health care cost trend rate assumed for next year	6.05%	6.50%
Ultimate health care cost trend rate	4.59%	5.00%
Year the ultimate rate is reached	2031	2026

PENSIONS AND OTHER POSTRETIRE_8

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Target Allocation and Actual Asset Allocations for Pension and VEBA Plan Assets (Details)	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	100.00%	100.00%
Target Allocation, Percentage, Next Fiscal Year	100.00%
Pension Plan [Member] | Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	51.80%	44.00%
Target Allocation, Percentage, Next Fiscal Year	41.30%
Pension Plan [Member] | Fixed Income Investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	33.80%	27.60%
Target Allocation, Percentage, Next Fiscal Year	39.70%
Pension Plan [Member] | Hedge Funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	2.20%	5.40%
Target Allocation, Percentage, Next Fiscal Year	5.00%
Pension Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	2.10%	6.60%
Target Allocation, Percentage, Next Fiscal Year	2.20%
Pension Plan [Member] | Structured Finance [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	5.00%	7.00%
Target Allocation, Percentage, Next Fiscal Year	5.20%
Pension Plan [Member] | Real Estate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	3.30%	9.40%
Target Allocation, Percentage, Next Fiscal Year	5.20%
Pension Plan [Member] | Absolute return fixed income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	1.80%	0.00%
Target Allocation, Percentage, Next Fiscal Year	1.40%
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	100.00%	100.00%
Target Allocation, Percentage, Next Fiscal Year	100.00%
Other Postretirement Benefits Plan [Member] | Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	22.20%	7.20%
Target Allocation, Percentage, Next Fiscal Year	20.30%
Other Postretirement Benefits Plan [Member] | Fixed Income Investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	66.40%	79.80%
Target Allocation, Percentage, Next Fiscal Year	69.60%
Other Postretirement Benefits Plan [Member] | Hedge Funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	1.80%	4.80%
Target Allocation, Percentage, Next Fiscal Year	1.10%
Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	0.40%	0.70%
Target Allocation, Percentage, Next Fiscal Year	1.40%
Other Postretirement Benefits Plan [Member] | Structured Finance [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	0.90%	2.10%
Target Allocation, Percentage, Next Fiscal Year	1.00%
Other Postretirement Benefits Plan [Member] | Real Estate [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	1.80%	5.40%
Target Allocation, Percentage, Next Fiscal Year	1.10%
Other Postretirement Benefits Plan [Member] | Absolute return fixed income
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of Plan Assets	6.50%	0.00%
Target Allocation, Percentage, Next Fiscal Year	5.50%

PENSIONS AND OTHER POSTRETIRE_9

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	$ 5,332	$ 749	$ 687
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	783	260	240
Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	2,431	515
Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	331	185
Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	807	22
Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	221	41
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	670	212	$ 229
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	38	34
Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	1,424	0
Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	193	0
Defined Benefit Plan, Equity Securities, US | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	1,950	169
Defined Benefit Plan, Equity Securities, US | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	119	12
Defined Benefit Plan, Equity Securities, US | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	1,163	169
Defined Benefit Plan, Equity Securities, US | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	26	12
Defined Benefit Plan, Equity Securities, US | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Defined Benefit Plan, Equity Securities, US | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Defined Benefit Plan, Equity Securities, US | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Defined Benefit Plan, Equity Securities, US | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Defined Benefit Plan, Equity Securities, US | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	787	0
Defined Benefit Plan, Equity Securities, US | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	93	0
International | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	810	161
International | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	55	7
International | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	615	161
International | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	6	7
International | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
International | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
International | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
International | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
International | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	195	0
International | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	49	0
US Government Agencies Debt Securities | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	476	33
US Government Agencies Debt Securities | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	156	0
US Government Agencies Debt Securities | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	141	11
US Government Agencies Debt Securities | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	62	0
US Government Agencies Debt Securities | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	295	22
US Government Agencies Debt Securities | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	94	0
US Government Agencies Debt Securities | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
US Government Agencies Debt Securities | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
US Government Agencies Debt Securities | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	40	0
US Government Agencies Debt Securities | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
US Corporate Bonds | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	1,281	174
US Corporate Bonds | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	364	207
US Corporate Bonds | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	512	174
US Corporate Bonds | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	237	166
US Corporate Bonds | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	466	0
US Corporate Bonds | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	127	41
US Corporate Bonds | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
US Corporate Bonds | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
US Corporate Bonds | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	303	0
US Corporate Bonds | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Non-US and Other Bonds | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	46	0
Non-US and Other Bonds | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Non-US and Other Bonds | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	46	0
Non-US and Other Bonds | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Non-US and Other Bonds | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	99	0
Absolute return fixed income | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	51	0
Absolute return fixed income | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Absolute return fixed income | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	99	0
Absolute return fixed income | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	51	0
Hedge Funds [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	118	40
Hedge Funds [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	14	12
Hedge Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Hedge Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Hedge Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Hedge Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	118	40
Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	14	12
Hedge Funds [Member] | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Hedge Funds [Member] | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Private Equity Funds [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	114	50
Private Equity Funds [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	3	2
Private Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	114	50
Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	3	2
Private Equity Funds [Member] | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Private Equity Funds [Member] | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Structured Credit [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	264	52
Structured Credit [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	7	6
Structured Credit [Member] | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Structured Credit [Member] | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Structured Credit [Member] | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Structured Credit [Member] | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Structured Credit [Member] | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	264	52
Structured Credit [Member] | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	7	6
Structured Credit [Member] | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Structured Credit [Member] | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Real Estate [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	174	70
Real Estate [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	14	14
Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Real Estate [Member] | Fair Value, Inputs, Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Real Estate [Member] | Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Real Estate [Member] | Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	174	70
Real Estate [Member] | Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	14	14
Real Estate [Member] | Fair Value Measured at Net Asset Value Per Share | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	0	0
Real Estate [Member] | Fair Value Measured at Net Asset Value Per Share | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Amount	$ 0	$ 0

PENSIONS AND POSTRETIREMENT BEN

PENSIONS AND POSTRETIREMENT BENEFITS - Fair Value Measurements Using Significant Unobservable Inputs (Level 3) on Changes in Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	$ 749	$ 687
Actual return on plan assets:
Fair value of plan assets — end of year	5,332	749
Pension Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	50
Actual return on plan assets:
Fair value of plan assets — end of year	114	50
Reserve for investment commitments	86
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	260	240
Actual return on plan assets:
Fair value of plan assets — end of year	783	260
Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	2
Actual return on plan assets:
Fair value of plan assets — end of year	3	2
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	212	229
Actual return on plan assets:
Relating to assets still held at the reporting date	8	(1)
Relating to assets sold during the period	6	30
Purchases	195	17
Sales	(13)	(60)
Acquired through business combinations	262
Other	0	(3)
Fair value of plan assets — end of year	670	212
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	50
Actual return on plan assets:
Fair value of plan assets — end of year	114	50
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	34
Actual return on plan assets:
Fair value of plan assets — end of year	38	34
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	2
Actual return on plan assets:
Fair value of plan assets — end of year	3	2
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member] | Veba Trust [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	34	36
Actual return on plan assets:
Relating to assets still held at the reporting date	2	1
Relating to assets sold during the period	1	0
Purchases	0	0
Sales	(1)	(3)
Acquired through business combinations	2
Other	0	0
Fair value of plan assets — end of year	$ 38	$ 34

PENSIONS AND OTHER POSTRETIR_10

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Defined Contribution Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postemployment Benefits [Abstract]
Defined contribution pension plans	$ 22	$ 3	$ 3

PENSIONS AND OTHER POSTRETIR_11

PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Multiemployer Plan (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Multiemployer Plan [Line Items]
Contributions	$ 30	$ 4	$ 4
Steelworkers Pension Trust
Multiemployer Plan [Line Items]
Pension Plan EIN	236648508
Pension Plan Number	499
Pension Protection Act Zone Status	Green	Green
FIP/RP Status Pending/Implemented	No
Contributions	$ 14	$ 4	$ 4
Surcharge Imposed	No
Multiemployer Plan, Pension, Significant, Accumulated Benefit Obligation		5,748
Multiemployer Plan, Pension, Significant, Plan Asset		$ 5,372
Multiemployer Plan, Pension, Significant, Funded Status Percentage		0.93
Steelworkers Pension Trust | ArcelorMittal USA and AK Steel [Member]
Multiemployer Plan [Line Items]
Multiemployer Plan, Pension, Significant, Contribution, Percentage		0.30	0.30
Steelworkers Pension Trust | Other Companies
Multiemployer Plan [Line Items]
Multiemployer Plan, Pension, Significant, Contribution, Percentage		0.05	0.05
Steelworkers Pension Trust | Minimum [Member]
Multiemployer Plan [Line Items]
Expiration Date of Collective Bargaining Agreement	Jan. 22, 2021
Steelworkers Pension Trust | Maximum [Member]
Multiemployer Plan [Line Items]
Expiration Date of Collective Bargaining Agreement	Oct. 1, 2022
IAM National Pension Fund's National Pension Plan
Multiemployer Plan [Line Items]
Pension Plan EIN	516031295
Pension Plan Number	002
Pension Protection Act Zone Status	Red	Green
FIP/RP Status Pending/Implemented	Implemented
Contributions	$ 16	$ 0	$ 0
Surcharge Imposed	Yes
IAM National Pension Fund's National Pension Plan | Minimum [Member]
Multiemployer Plan [Line Items]
Expiration Date of Collective Bargaining Agreement	May 31, 2022
IAM National Pension Fund's National Pension Plan | Maximum [Member]
Multiemployer Plan [Line Items]
Expiration Date of Collective Bargaining Agreement	May 15, 2023
American Maritime Officers Plan
Multiemployer Plan [Line Items]
Pension Plan EIN	131936709
Pension Plan Number	001
Pension Protection Act Zone Status	Green	Green
FIP/RP Status Pending/Implemented	No
Contributions	$ 0	$ 0	$ 0
Surcharge Imposed	No
Expiration Date of Collective Bargaining Agreement	Jul. 31, 2021

STOCK COMPENSATION PLANS - Narr

STOCK COMPENSATION PLANS - Narrative (Details)	Mar. 13, 2020	Dec. 31, 2020shares	Dec. 31, 2019shares	Dec. 31, 2018shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		2,010,277	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		4,900,000
AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		5,700,000
Business Acquisition, Shares Exchange Ratio	0.400
Granted shares		2,000,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		2,510,853	572,104	675,599
Performance Shares [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		500,000
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		1,160,928	572,104	685,599
Restricted Stock Units (RSUs) [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		300,000
Long-term Performance Plan | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		1,000,000
Restricted Stock [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares		200,000

STOCK COMPENSATION PLANS - Stoc

STOCK COMPENSATION PLANS - Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Costs and Expenses	$ (17)	$ (18)	$ (15)
Share-based Payment Arrangement, Expense, Tax Benefit	4	4	0
Share-based Payment Arrangement, Expense, after Tax	$ (13)	$ (14)	$ (15)
Decrease in basic earnings per common share	$ (0.32)	$ 1.06	$ 3.80
Decrease in diluted earnings per common share	$ (0.32)	1.03	3.71
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount	$ 14
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 8 months 12 days
Share-based Payment Arrangement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Decrease in basic earnings per common share	$ (0.03)	(0.05)	(0.05)
Decrease in diluted earnings per common share	$ (0.03)	$ (0.05)	$ (0.05)
Cost of goods sold
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Costs and Expenses	$ (2)	$ (2)	$ (2)
Selling, General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Costs and Expenses	(13)	(16)	(13)
Acquisition-related Costs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Costs and Expenses	$ (2)	$ 0	$ 0

STOCK COMPENSATION PLANS - Perf

STOCK COMPENSATION PLANS - Performance Shares (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at beginning of year	1,935,878	1,424,723	1,848,312
Outstanding shares, weighted average grant date fair value	$ 10.34	$ 15.58	$ 14.46	$ 13.42
Granted shares	2,510,853	572,104	675,599
Granted Shares Weighted Average Fair Value	$ 5.49	$ 18.31	$ 11.93
Distributed	(1,938,786)	0	(489,953)
Distributed shares, weighted average grant date fair value	$ 12.23	$ 0	$ 10.89
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Conditions Adjustment in Period	549,154	0	(560,720)
Performance adjustment, weighted average grant date fair value	$ 15.63	$ 0	$ 10.69
Forfeited/canceled shares	(604,873)	(60,949)	(48,515)
Forfeited/canceled shares, weighted average grant date fair value	$ 5.70	$ 15.12	$ 19.33
Outstanding at end of year	2,452,226	1,935,878	1,424,723
Performance/vesting period	3 years
Performance Shares [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	0.00%
Performance Shares [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	200.00%
Performance Shares [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted shares	500,000
Distributed shares, weighted average grant date fair value	$ 4.59
Outstanding at end of year	100,000
Long-term Performance Plan | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted shares	1,000,000
Outstanding at end of year	300,000

STOCK COMPENSATION PLANS - Rest

STOCK COMPENSATION PLANS - Restricted Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at beginning of year	2,147,183	4,694,360	4,403,112
Outstanding shares, weighted average grant date fair value	$ 7.12	$ 9.10	$ 4.18	$ 3.64
Granted shares	1,160,928	572,104	685,599
Granted Shares Weighted Average Fair Value	$ 4.87	$ 11.24	$ 7.53
Distributed	(1,101,115)	(3,058,307)	(254,196)
Distributed shares, weighted average grant date fair value	$ 8.58	$ 1.95	$ 3.30
Forfeited/canceled shares	(63,413)	(60,974)	(140,155)
Forfeited/canceled shares, weighted average grant date fair value	$ 7.31	$ 9.31	$ 5.17
Outstanding at end of year	2,143,583	2,147,183	4,694,360
Performance/vesting period	3 years
Restricted Stock Units (RSUs) [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted shares	300,000
Distributed shares, weighted average grant date fair value	$ 4.87
Restricted Stock [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted shares	200,000

STOCK COMPENSATION PLANS - St_2

STOCK COMPENSATION PLANS - Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at beginning of year	563,230	563,230	599,870
Granted shares	2,010,277	0	0
Granted shares, weighted average exercise price	$ 11.86	$ 0	$ 0
Exercised, number of shares	(79,973)	0	(36,640)
Exercised, weighted average exercise price	$ 7.01	$ 0	$ 7.70
Forfeited/canceled, shares	(7,726)	0	0
Forfeitured/canceled, weighted average exercise price	$ 41.04	$ 0	$ 0
Outstanding at end of year	2,485,808	563,230	563,230
Outstanding, weighted average exercise price	$ 11.60	$ 10.42	$ 10.42	$ 10.25
Exercisable, shares	2,172,052	563,230	563,230
Exercisable, weighted average exercise price	$ 11.86	$ 10.42	$ 10.42
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	3 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 11
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	2 years 6 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 9
AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Granted shares	2,000,000
Granted shares, weighted average exercise price	$ 0.51

STOCK COMPENSATION PLANS - None

STOCK COMPENSATION PLANS - Nonemployee Directors (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount	$ 14,000,000
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares	1,160,928	572,104	685,599
Restricted Stock Units (RSUs) [Member] | AK Steel [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares	300,000
Share-based Payment Arrangement, Nonemployee [Member] | Director [Member] | Directors Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares	253,809	86,477	92,718
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount	$ 100,000
Share-based Payment Arrangement, Nonemployee [Member] | Director [Member] | Deferred Compensation, Share-based Payments [Member] | Directors Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted shares	54,794	23,659	17,170

INCOME TAXES - Schedule of Inco

INCOME TAXES - Schedule of Income Before Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ (201)	$ 313	$ 565
Income (Loss) from Continuing Operations before Income Taxes, Foreign	8	0	0
Income from continuing operations before income taxes	$ (193)	$ 313	$ 565

INCOME TAXES - Schedule of Comp

INCOME TAXES - Schedule of Components of Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Current Federal Tax Expense (Benefit)	$ (2)	$ (1)	$ (1)
Current State and Local Tax Expense (Benefit)	0	0	0
Current Foreign Tax Expense (Benefit)	(1)	0	1
Current Income Tax Expense (Benefit)	(3)	(1)	0
Deferred Federal Income Tax Expense (Benefit)	(95)	19	(475)
Deferred State and Local Income Tax Expense (Benefit)	(11)	0	0
Deferred Foreign Income Tax Expense (Benefit)	(2)	0	0
Income Tax Expense (Benefit)	$ (111)	$ 18	$ (475)

INCOME TAXES - Schedule of Effe

INCOME TAXES - Schedule of Effective Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ (41)	$ 66	$ 119
Income Tax Reconciliation Tax Deduction for percentage depletion in excess of cost depletion	(42)	(49)	(55)
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount	(9)	0	0
Effective Income Tax Rate Reconciliation, Disposition of Business, Amount	0	846	162
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	(11)	0	0
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	(8)	1	2
Income Tax Expense (Benefit)	$ (111)	$ 18	$ (475)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%
Income Tax Reconciliation Tax Deduction for percentage depletion in excess of cost depletion percent	22.00%	(16.00%)	(10.00%)
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Percent	4.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Disposition of Business, Percent	0.00%	271.00%	29.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	6.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	4.00%	1.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	57.00%	6.00%	(84.00%)
Current year activity [Member]
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 0	$ 0	$ (80)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	0.00%	0.00%	(14.00%)
valuation allowance reversal [Member]
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 0	$ 0	$ (461)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	0.00%	0.00%	(81.00%)
Dissolution of entities [Member]
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 0	$ (846)	$ (162)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	0.00%	(271.00%)	(29.00%)

INCOME TAXES - Components of In

INCOME TAXES - Components of Income Taxes for Other than Continuing Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	$ (52)	$ 11	$ 4
Other Comprehensive Income Unrealized Gain Loss On Mark To Market Adjustments Arising During Period Tax	(1)	0	1
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	$ (53)	$ 11	$ 5

INCOME TAXES - Schedule of Defe

INCOME TAXES - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Deferred Tax Assets [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards	$ 1,236	$ 795
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions	228	114
Deferred Tax Assets, Property, Plant and Equipment	0	1
Deferred Tax Assets, State Taxes	132	71
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Other	190	70
Deferred Tax Assets, Gross	1,786	1,051
Deferred Tax Assets, Valuation Allowance	(836)	(441)	$ (1,287)	$ (1,983)
Deferred Tax Assets, Net of Valuation Allowance	950	610
Components of Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities Investment In Ventures	(144)	(137)
Deferred Tax Liabilities, Property, Plant and Equipment	246	0
Deferred Tax Liabilities Other Assets	(68)	(14)
Deferred Tax Liabilities, Net	(458)	(151)
Deferred Tax Assets, Net	$ 492	$ 459

INCOME TAXES Income Taxes (Sche

INCOME TAXES Income Taxes (Schedule of Valuation Allowance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Valuation Allowance [Line Items]
Deferred Tax Assets, Valuation Allowance	$ 836	$ 441	$ 1,287	$ 1,983
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount			696
Included in income tax expense (benefit) [Member]
Valuation Allowance [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	(3)	(846)	(691)
Change in deferred assets in other comprehensive income [Member]
Valuation Allowance [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	0	0	(5)
Acquisition of noncontrolling interest [Member]
Valuation Allowance [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ 398	$ 0	$ 0

INCOME TAXES (Narrative) (Detai

INCOME TAXES (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	$ 7,444	$ 3,459
Deferred Tax Assets, Operating Loss Carryforwards, Foreign	1,592	1,592
Deferred Tax Assets, Valuation Allowance	836	441	$ 1,287	$ 1,983
Deferred Tax Asset, Interest Carryforward	55
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount			696
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	6
Estimated Taxable Income to Fully Utilize Deferred Tax Assets	10
Undistributed Earnings of Foreign Subsidiaries	0	0
valuation allowance reversal [Member]
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	0	0	(461)
Dissolution of entities [Member]
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	0	(846)	$ (162)
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Deferred Tax Assets, Valuation Allowance	98	38
LUXEMBOURG
Income Tax Contingency [Line Items]
Deferred Tax Assets, Valuation Allowance	397	397
LUXEMBOURG | Dissolution of entities [Member]
Income Tax Contingency [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	(1,300)
LUXEMBOURG | Tax Law Change [Member]
Income Tax Contingency [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		$ 46
Maximum [Member] | LUXEMBOURG
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent		24.94%	26.01%
Intercompany notes [Member] | LUXEMBOURG
Income Tax Contingency [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ 447

INCOME TAXES (Schedule of Unrec

INCOME TAXES (Schedule of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 107	$ 29	$ 29	$ 33
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	7	0	4
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	(4)	0	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0	0	(8)
Other Increases decreases to unrecognized tax benefits	75	0	$ 0
Other Noncurrent Liabilities [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	2	4
Other Current Liabilities [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	9
Other Liabilities [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	11
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	1	$ 4
Other Noncurrent Assets [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits	$ 96

LEASE OBLIGATIONS - Lease Costs

LEASE OBLIGATIONS - Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating Lease, Payments	$ 43	$ 4
Finance Lease, Right-of-Use Asset, Amortization	15	6
Finance Lease, Interest Expense	4	2
Short-term Lease, Cost	13	6
Lease, Cost	$ 75	$ 18

LEASE OBLIGATIONS - Other infor

LEASE OBLIGATIONS - Other information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating Lease, Payments	$ 43	$ 4
Finance Lease, Interest Payment on Liability	4	2
Finance Lease, Principal Payments	15	6
Right-of-use asset - finance leases	$ 44	$ 29	$ 8
Operating Lease, Weighted Average Remaining Lease Term	8 years	10 years
Finance Lease, Weighted Average Remaining Lease Term	5 years	6 years
Operating Lease, Weighted Average Discount Rate, Percent	8.00%	8.00%
Finance Lease, Weighted Average Discount Rate, Percent	4.00%	5.00%

LEASE OBLIGATIONS - Future Mini

LEASE OBLIGATIONS - Future Minimum Lease Payments (Details) $ in Millions	Dec. 31, 2020USD ($)
Leases [Abstract]
Finance Lease, Liability, Payments, Due in Next Rolling Twelve Months	$ 100
Lessee, Operating Lease, Liability, Payments, Due Next Rolling Twelve Months	70
Finance Lease, Liability, Payments, Due in Rolling Year Two	95
Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Two	53
Finance Lease, Liability, Payments, Due in Rolling Year Three	78
Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Three	46
Finance Lease, Liability, Payments, Due in Rolling Year Four	23
Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Four	38
Finance Lease, Liability, Payments, Due in Rolling Year Five	22
Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Five	34
Finance Lease, Liability, Payments, Due in Rolling after Year Five	76
Lessee, Operating Lease, Liability, Payments, Due after Rolling Year Five	122
Finance Lease, Liability, Payment, Due	394
Lessee, Operating Lease, Liability, to be Paid	363
Finance Lease, Liability, Undiscounted Excess Amount	59
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	99
Finance Lease, Liability	$ 335
Finance Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities
Operating Lease, Liability	$ 264
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities
Finance Lease, Liability, Current	$ 91
Finance Lease, Liability, Noncurrent	$ 244
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Current	$ 53
Operating Lease, Liability, Noncurrent	$ 211
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent

ASSET RETIREMENT OBLIGATIONS -

ASSET RETIREMENT OBLIGATIONS - Summary Of Asset Retirement Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Environmental Exit Cost [Line Items]
Asset Retirement Obligation	$ 342	$ 165	$ 172
Asset Retirement Obligation, Current	7	2
Asset Retirement Obligations, Noncurrent	335	163
Operating Segments [Member]
Environmental Exit Cost [Line Items]
Asset Retirement Obligation	$ 190	$ 22

ASSET RETIREMENT OBLIGATIONS _2

ASSET RETIREMENT OBLIGATIONS - Asset Retirement Obligation Roll-forward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation [Roll Forward]
Asset Retirement Obligation at Beginning of Year	$ 165	$ 172
Asset Retirement Obligation, Liabilities Assumed, Acquisition	172	0
Asset Retirement Obligation, Accretion Expense	14	10
Asset Retirement Obligation, Liabilities Settled	(9)	(1)
Asset Retirement Obligation, Revision of Estimate		(16)
Asset Retirement Obligation at End of Year	$ 342	$ 165

DERIVATIVE INSTRUMENTS AND HEDG

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Customer Supply Agreements [Member] | Sales [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 122	$ 78	$ 426

CAPITAL STOCK - Narrative (Deta

CAPITAL STOCK - Narrative (Details) $ / shares in Units, $ in Millions	Dec. 09, 2020shares	Mar. 13, 2020USD ($)	Dec. 31, 2020USD ($)shareMultiple$ / sharesshares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)	Apr. 24, 2019USD ($)	Nov. 26, 2018USD ($)
Class of Stock [Line Items]
Common shares, par value | $ / shares			$ 0.125
Temporary Equity, Shares Issued			583,273
Temporary Equity, Shares Outstanding			583,273
Stock Repurchase Program, Authorized Amount | $						$ 100	$ 200
Repurchase of common shares | $			$ 0	$ 253	$ 48
Preferred Stock, Share Multiple | shareMultiple			100
AK Steel [Member]
Class of Stock [Line Items]
Business Acquisition, Shares Exchange Ratio		0.400
Common Stock [Member]
Class of Stock [Line Items]
Common shares, par value | $ / shares			$ 0.125
Stock Repurchased During Period, Shares				24,000,000
Common Stock [Member] | AK Steel [Member]
Class of Stock [Line Items]
Business Acquisition, Shares Exchange Ratio			0.400
Stock Issued During Period, Shares, Acquisitions			127,000,000
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned | $		$ 618
Common Stock [Member] | ArcelorMittal USA [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, Acquisitions	78,186,671		78,000,000
Series A Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized			3,000,000
Preferred Stock, Shares Issued			0
Preferred Stock, Shares Outstanding			0
Series B Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Shares Authorized			4,000,000
Temporary Equity, Shares Issued			583,273
Temporary Equity, Shares Outstanding			583,273
Series B Preferred Stock | ArcelorMittal USA [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, Acquisitions	583,273		583,273
Series B Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage, Daily Compounding Rate			10.00%
Preferred Stock, Dividend Rate, Percentage			2.00%

CAPITAL STOCK - Dividends Decla

CAPITAL STOCK - Dividends Declared (Details) - $ / shares	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Dividends Payable [Line Items]
Common stock dividends paid, per share	$ 0.06	$ 0.06	$ 0.10	$ 0.06	$ 0.05	$ 0.05
Common Stock, Special Dividends, Per Share, Cash Paid			$ 0.04

ACCUMULATED OTHER COMPREHENSI_2

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - AOCI (Loss) and Related Tax Effects (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Including Portion Attributable to Noncontrolling Interest, before Tax	$ (219)	$ (458)	$ (412)
AOCI Including Portion Attributable to Noncontrolling Interest, Tax	86	139	128
Accumulated other comprehensive loss	(133)	(319)	(284)	$ (39)
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Including Portion Attributable to Noncontrolling Interest, before Tax	(221)	(454)	(408)
AOCI Including Portion Attributable to Noncontrolling Interest, Tax	86	138	127
Accumulated other comprehensive loss	(135)	(316)	(281)	264
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Including Portion Attributable to Noncontrolling Interest, before Tax	3
AOCI Including Portion Attributable to Noncontrolling Interest, Tax	0
Accumulated other comprehensive loss	3	0	0	225
Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Including Portion Attributable to Noncontrolling Interest, before Tax	(1)	(4)	(4)
AOCI Including Portion Attributable to Noncontrolling Interest, Tax	0	1	1
Accumulated other comprehensive loss	$ (1)	$ (3)	$ (3)	$ 0

ACCUMULATED OTHER COMPREHENSI_3

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)

ACCUMULATED OTHER COMPREHENSI_4

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Details of AOCI (Loss) Components Reclassified from Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Gain on foreign currency translation	$ (3)	$ 0	$ 225
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Prior service costs	(1)	(1)	(1)
Net actuarial loss	(30)	(29)	(27)
Settlements	6	0	0
Total before taxes	23	28	26
Income tax expense	(5)	(6)	0
Net of taxes	18	22	26
Gain on foreign currency translation	0	0	(228)
Commodity contracts	10	3	(2)
Income tax expense	(2)	(1)	0
Net of taxes	8	2	(2)
Total reclassifications for the period, net of tax	$ 26	$ (24)	$ 204

RELATED PARTIES - Narrative (De

RELATED PARTIES - Narrative (Details) - Hibbing [Member]	Dec. 31, 2020	Dec. 09, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Ownership interest, equity method investment	85.30%	62.30%	23.00%
ArcelorMittal USA [Member]
Related Party Transaction [Line Items]
Ownership interest, equity method investment		62.30%
U. S. Steel Canada [Member]
Related Party Transaction [Line Items]
Ownership interest, equity method investment	14.70%

RELATED PARTIES - Summary Of Re

RELATED PARTIES - Summary Of Related Party Transactions Table Disclosure (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Revenue from related parties	$ 893	$ 1,015	$ 1,344
Revenues	$ 5,354	$ 1,990	$ 2,332
Related party product revenue as a percent of total product revenue	16.70%	51.00%	57.60%
Purchases from related parties	$ 16	$ 0	$ 0
Realization of Deferred Revenue	$ 35	$ 0	$ 0

RELATED PARTIES - Classificatio

RELATED PARTIES - Classification of Related Party Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 2	$ 31
Due from Related Parties, Current	0	45
Accounts Payable, Related Parties, Current	(6)	0
Due to Related Parties, Current	$ 0	$ (2)

VARIABLE INTEREST ENTITIES (Det

VARIABLE INTEREST ENTITIES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Net Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest	$ 41	$ 0	$ 0
Cash and cash equivalents	112	353
Inventories	3,828	317
Property, plant and equipment, net	8,743	1,929	$ 1,286
Accounts Payable, Current	(1,575)	(193)
Stockholders' Equity Attributable to Noncontrolling Interest	(323)	$ 0
Variable Interest Entity, Primary Beneficiary [Member] | SunCoke Middletown [Member]
Variable Interest Entity [Line Items]
Net Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest	41
Cash and cash equivalents	5
Inventories	21
Property, plant and equipment, net	308
Accounts Payable, Current	(15)
Other Assets (Liabilities), Net	(10)
Stockholders' Equity Attributable to Noncontrolling Interest	$ (309)

EARNINGS PER SHARE - Earnings P

EARNINGS PER SHARE - Earnings Per Share Computation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Income (loss) from continuing operations	$ (82)	$ 295	$ 1,040
Net Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest	41	0	0
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(123)	295	1,040
Income (loss) from discontinued operations, net of tax	1	(2)	88
Net Income (Loss) Attributable to Parent	$ (122)	$ 293	$ 1,128
Weighted average number of shares:
Basic	379	277	297
Convertible senior notes	0	4	3
Employee stock plans	0	3	4
Diluted	379	284	304
Earnings (loss) per common share attributable to Cliffs common shareholders - basic:
Continuing operations	$ (0.32)	$ 1.07	$ 3.50
Discontinued operations	0	(0.01)	0.30
Earnings (loss) per common share attributable to Cliffs common shareholders - basic	(0.32)	1.06	3.80
Earnings (loss) per common share attributable to Cliffs common shareholders - diluted:
Continuing operations	(0.32)	1.04	3.42
Discontinued operations	0	(0.01)	0.29
Earnings (loss) per common share attributable to Cliffs common shareholders - diluted	$ (0.32)	$ 1.03	$ 3.71

EARNINGS PER SHARE - Anti-Dilut

EARNINGS PER SHARE - Anti-Dilutive Securities (Details) shares in Millions	12 Months Ended
	Dec. 31, 2020shares
Series B Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	4
Convertible Debt Securities [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	2
Share-based Payment Arrangement [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Environmental Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies	$ 135	$ 2
Accrued Environmental Loss Contingencies, Current	18	0
Accrued Environmental Loss Contingencies, Noncurrent	$ 117	$ 2

SUBSEQUENT EVENTS SUBSEQUENT EV

SUBSEQUENT EVENTS SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions

Mar. 12, 2021

Mar. 11, 2021

Dec. 09, 2020

Apr. 24, 2020

Mar. 27, 2020

Mar. 13, 2020

Mar. 31, 2021

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Feb. 17, 2021

Feb. 11, 2021

Jun. 19, 2020

Apr. 17, 2020

Mar. 16, 2020

Subsequent Event [Line Items]

Common shares, issued (in shares)

506,832,537

301,886,794

Debt Instrument, Par Value

$ 5,595

Debt Extinguished

$ 736

1,513

$ 724

$ 227

Subsequent Event [Member]

Subsequent Event [Line Items]

Common shares, issued (in shares)

20,000,000

Common Stock, Shares, Option to Purchase Additional Shares

9,000,000

Arcelor Mittal [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Common Stock, Shares, Sold

40,000,000

AK Steel [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

132

9.875% 2025 Senior Secured Notes | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Extinguished

$ 334

$ 334

9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

$ 555

$ 955

$ 400

9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

109.875%

Debt Extinguished

$ 322

9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

109.875%

5.875% 2027 Senior Notes

Subsequent Event [Line Items]

Debt Extinguished

$ 195

5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

$ 556

750

5.875% 2027 Senior Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

105.875%

6.75% 2026 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

$ 725

$ 845

$ 120

6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

106.75%

4.625% 2029 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

$ 500

4.875% 2031 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

$ 500

4.875% 2024 Senior Secured Notes

Subsequent Event [Line Items]

Debt Extinguished

$ 6

4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

395

$ 400

4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

102.438%

Debt Extinguished

$ 395

6.375% 2025 Senior Notes

Subsequent Event [Line Items]

Debt Extinguished

168

6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

64

$ 232

6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

103.188%

Debt Extinguished

$ 64

7.625% 2021 AK Senior Notes

Subsequent Event [Line Items]

Debt Extinguished

$ 9

364

373

7.625% 2021 AK Senior Notes | AK Steel [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

34

7.625% 2021 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

100.00%

Debt Extinguished

$ 34

7.50% 2023 AK Senior Notes [Member]

Subsequent Event [Line Items]

Debt Extinguished

$ 56

$ 311

367

7.50% 2023 AK Senior Notes [Member] | AK Steel [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

13

7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

101.875%

Debt Extinguished

$ 13

6.375% 2025 AK Senior Notes

Subsequent Event [Line Items]

Debt Extinguished

9

6.375% 2025 AK Senior Notes | AK Steel [Member]

Subsequent Event [Line Items]

Debt Instrument, Par Value

$ 29

6.375% 2025 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Debt Instrument, Redemption Price, Percentage

103.188%

Debt Extinguished

$ 29

Common Stock [Member] | Subsequent Event [Member]

Subsequent Event [Line Items]

Common stock, shares issued, price per share

$ 16.12

ArcelorMittal USA [Member] | Common Stock [Member]

Subsequent Event [Line Items]

Stock Issued During Period, Shares, Acquisitions

78,186,671

78,000,000