DEBT AND CREDIT FACILITIES - Schedule of Extinguishment of Debt (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Mar. 11, 2021	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 914		$ 1,274
Gain (Loss) on Extinguishment of Debt		(66)	$ (22)	(80)	$ (88)
9.875% 2025 Senior Secured Notes
Extinguishment of Debt [Line Items]
Debt Extinguished	$ 322	607		607
Gain (Loss) on Extinguishment of Debt		$ (85)		$ (85)
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc.
Extinguishment of Debt [Line Items]
Stated interest rate		9.875%		9.875%
6.750% 2026 Senior Secured Notes
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 16		$ 16
Gain (Loss) on Extinguishment of Debt		$ (1)		$ (1)
6.750% 2026 Senior Secured Notes | Cleveland-Cliffs Inc.
Extinguishment of Debt [Line Items]
Stated interest rate		6.75%		6.75%
1.500% 2025 Convertible Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 0		$ 294
Gain (Loss) on Extinguishment of Debt		$ 0		$ (16)
1.500% 2025 Convertible Senior Notes | Cleveland-Cliffs Inc.
Extinguishment of Debt [Line Items]
Stated interest rate		1.50%		1.50%
4.625% 2029 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 130		$ 130
Gain (Loss) on Extinguishment of Debt		$ 8		$ 8
4.625% 2029 Senior Notes | Cleveland-Cliffs Inc.
Extinguishment of Debt [Line Items]
Stated interest rate		4.625%		4.625%
4.875% 2031 Senior Notes
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 161		$ 161
Gain (Loss) on Extinguishment of Debt		$ 12		$ 12
4.875% 2031 Senior Notes | Cleveland-Cliffs Inc.
Extinguishment of Debt [Line Items]
Stated interest rate		4.875%		4.875%
Industrial Revenue Bonds
Extinguishment of Debt [Line Items]
Debt Extinguished		$ 0		$ 66
Gain (Loss) on Extinguishment of Debt		$ 0		$ 2