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	Jun. 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, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	$ 2,326.3	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, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total long-term debt, fair value	2,104.5	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	406.0	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	454.2	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	472.1	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	730.8	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	263.2	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	392.8	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	240.9	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	463.3	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	714.6	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