Earnings and Dividends Per Common Share - Computations for Basic and Diluted Income (Loss) Per Common Share from Continuing Operations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Earnings (loss) attributable to United States Steel Corporation stockholders	$ 1,012	$ (589)	$ 1,103	$ (980)
Weighted-average shares outstanding (in thousands):
Basic (in shares)	269,872	175,327	259,668	172,775
Effect of Senior Convertible Notes (in shares)	11,975	0	10,439	0
Effect of stock options, restricted stock units and performance awards (in shares)	4,490	0	4,405	0
Adjusted weighted-average shares outstanding, diluted (in shares)	286,337	175,327	274,512	172,775
Basic (loss) earnings per common share (in dollars per share)	$ 3.75	$ (3.36)	$ 4.25	$ (5.67)
Diluted (loss) earnings per common share (in dollars per share)	$ 3.53	$ (3.36)	$ 4.02	$ (5.67)