Cleveland-Cliffs Inc. Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE MEASUREMENTS (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Details)

FAIR VALUE MEASUREMENTS (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Interest Rate Swap [Member]
Long-term debt:
Fair Value Adjustment to Interest Rate Hedge	$ 1.5	$ 1.9
Carrying Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Long-term debt:
Total long-term debt, fair value	1,689.4	2,192.6
Carrying Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Long-term debt:
Fair Value Adjustment to Interest Rate Hedge	1.5	1.9
Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Long-term debt:
Total long-term debt, fair value	1,620.2	2,224.6
Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Long-term debt:
Fair Value Adjustment to Interest Rate Hedge	1.5	1.9
Senior Notes [Member] \| $540 Million 8.25% 2020 Lien Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	540.0	540.0
Senior Notes [Member] \| $218.5 Million 8.00% 2020 Lien Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	0.0	218.5
Senior Notes [Member] \| $544 Million 7.75% 2020 Lien Notes[Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	0.0	544.2
Senior Notes [Member] \| $1,075 Million 5.75% 2025 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	1,075.0	0.0
Senior Notes [Member] \| $400 Million 5.90% 2020 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	400.0	400.0
Senior Notes [Member] \| $500 million 4.80% 2020 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	500.0	500.0
Senior Notes [Member] \| $700 Million 4.875% 2021 Senior Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	700.0	700.0
Senior Notes [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Original Face Value	800.0	800.0
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $540 Million 8.25% 2020 Lien Notes [Member]
Long-term debt:
Total long-term debt, fair value	0.0	506.3
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $544 Million 7.75% 2020 Lien Notes[Member]
Long-term debt:
Total long-term debt, fair value	0.0	339.1
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $1,075 Million 5.75% 2025 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	1,046.8	0.0
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 5.90% 2020 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	88.5	224.5
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $500 million 4.80% 2020 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	122.0	235.9
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $700 Million 4.875% 2021 Senior Note [Member]
Long-term debt:
Total long-term debt, fair value	138.0	308.2
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	292.6	292.5
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| $218.5 Million 8.00% 2020 Lien Notes [Member]
Long-term debt:
Total long-term debt, fair value	0.0	284.2
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $540 Million 8.25% 2020 Lien Notes [Member]
Long-term debt:
Total long-term debt, fair value	0.0	595.0
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $544 Million 7.75% 2020 Lien Notes[Member]
Long-term debt:
Total long-term debt, fair value	0.0	439.7
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $1,075 Million 5.75% 2025 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	1,032.0	0.0
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 5.90% 2020 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	88.4	219.6
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $500 million 4.80% 2020 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	116.9	221.1
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $700 Million 4.875% 2021 Senior Note [Member]
Long-term debt:
Total long-term debt, fair value	132.4	283.1
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Long-term debt:
Total long-term debt, fair value	249.0	234.7
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| $218.5 Million 8.00% 2020 Lien Notes [Member]
Long-term debt:
Total long-term debt, fair value	0.0	229.5
Line of Credit [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Revolving Credit Facility [Member]
Long-term debt:
Total long-term debt, fair value	0.0	0.0
Line of Credit [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Revolving Credit Facility [Member]
Long-term debt:
Total long-term debt, fair value	$ 0.0	$ 0.0