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

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

Amount of contribution received by defined benefit plan from employer which increases plan assets.