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