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

Annual report pursuant to Section 13 and 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Details)

FAIR VALUE OF FINANCIAL INSTRUMENTS (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Reported Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Long-term debt:
Long-term Debt, Fair Value	$ 2,304.2	$ 2,192.6
Reported Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Long-term debt:
Fair Value Adjustment to Interest Rate Hedge	1.4	1.9
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Long-term debt:
Long-term Debt, Fair Value	2,346.3	2,224.6
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Long-term debt:
Fair Value Adjustment to Interest Rate Hedge	1.4	1.9
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| $218.5 Million 8.00% 2020 1.5 Lien Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $540 Million 8.25% 2020 First Lien Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0	506.3
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $544.2 Million 7.75% 2020 Second Lien Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0	339.1
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 5.90% 2020 Senior Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	88.6	224.5
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $500 Million 4.80% 2020 Senior Notes[Member]
Long-term debt:
Long-term Debt, Fair Value	122.0	235.9
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $700 Million 4.875% 2021 Senior Notes[Member]
Long-term debt:
Long-term Debt, Fair Value	138.0	308.2
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $1.075 Billion 5.75% 2025 Senior Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	1,047.2	0.0
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	292.6	292.5
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $316 Million 1.5% 2025 Senior Notes [Domain]
Long-term debt:
Long-term Debt, Fair Value	224.1	0.0
Senior Notes [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 4.875% 2024 Senior Notes [Domain]
Long-term debt:
Long-term Debt, Fair Value	390.3	0.0
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| $218.5 Million 8.00% 2020 1.5 Lien Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0	229.5
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $540 Million 8.25% 2020 First Lien Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0	595.0
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $544.2 Million 7.75% 2020 Second Lien Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0	439.7
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 5.90% 2020 Senior Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	88.0	219.6
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $500 Million 4.80% 2020 Senior Notes[Member]
Long-term debt:
Long-term Debt, Fair Value	118.8	221.1
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $700 Million 4.875% 2021 Senior Notes[Member]
Long-term debt:
Long-term Debt, Fair Value	130.8	283.1
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $1.075 Billion 5.75% 2025 Senior Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	1,029.3	0.0
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Long-term debt:
Long-term Debt, Fair Value	227.1	234.7
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $316 Million 1.5% 2025 Senior Notes [Domain]
Long-term debt:
Long-term Debt, Fair Value	352.9	0.0
Senior Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 4.875% 2024 Senior Notes [Domain]
Long-term debt:
Long-term Debt, Fair Value	398.0	0.0
Line of Credit [Member] \| Reported Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| ABL Facility [Member]
Long-term debt:
Long-term Debt, Fair Value	0.0	0.0
Line of Credit [Member] \| Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| ABL Facility [Member]
Long-term debt:
Long-term Debt, Fair Value	$ 0.0	$ 0.0