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

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, 2019	Dec. 31, 2018
Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Adjustment to Interest Rate Hedge		$ 0.2
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	$ 2,187.5	2,039.9
Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Adjustment to Interest Rate Hedge	0.0	0.2
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	2,109.1	2,092.9
Carrying Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Adjustment to Interest Rate Hedge	0.0	0.2
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 4.875% 2024 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	408.9	370.2
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $700 Million 4.875% 2021 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	0.0	122.3
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $316.25 Million 1.50% 2025 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	345.8	352.4
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $1.075 Billion 5.75% 2025 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	466.2	962.0
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $750 Million 5.875% 2027 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	712.2	0.0
Senior Notes [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	254.4	232.8
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $400 Million 4.875% 2024 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	393.2	392.1
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $700 Million 4.875% 2021 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	0.0	123.8
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $316.25 Million 1.50% 2025 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	243.8	235.2
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $1.075 Billion 5.75% 2025 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	463.7	1,048.8
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $750 Million 5.875% 2027 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	715.5	0.0
Senior Notes [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 1 [Member] \| $800 Million 6.25% 2040 Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	292.9	292.8
Line of Credit [Member] \| Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Revolving Credit Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	0.0	0.0
Line of Credit [Member] \| Carrying Value [Member] \| Fair Value, Inputs, Level 2 [Member] \| Revolving Credit Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	$ 0.0	$ 0.0