Quarterly report pursuant to Section 13 or 15(d)

DEBT AND CREDIT FACILITIES - Schedule of Extinguishment of Debt (Details)

v3.20.2
DEBT AND CREDIT FACILITIES - Schedule of Extinguishment of Debt (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 01, 2020
Apr. 24, 2020
Mar. 27, 2020
Mar. 13, 2020
Sep. 30, 2020
Jun. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount   $ 736.4           $ 1,487.5 $ 724.0
Reduction of Principal Long-Term Debt   $ 181.2              
Gain (loss) on extinguishment of debt         $ 0.0   $ 0.0 132.6 (18.2)
Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount           $ 11.2      
7.625% 2021 AK Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount     $ 8.5 $ 364.2       372.7  
Gain (loss) on extinguishment of debt               0.4  
7.50% 2023 AK Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount     $ 56.5 $ 310.7       367.2  
Gain (loss) on extinguishment of debt               2.8  
4.875% 2024 Senior Secured Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               5.5  
Gain (loss) on extinguishment of debt               0.5  
6.375% 2025 Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               167.5  
Gain (loss) on extinguishment of debt               21.3  
1.50% 2025 Convertible Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               20.0  
Gain (loss) on extinguishment of debt               1.3  
4.875% 2021 Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount                 124.0
Gain (loss) on extinguishment of debt                 (5.3)
5.75% 2025 Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               77.1 600.0
Gain (loss) on extinguishment of debt               16.3 $ (12.9)
7.00% 2027 Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               247.3  
Gain (loss) on extinguishment of debt               28.4  
5.875% 2027 Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               194.5  
Gain (loss) on extinguishment of debt               48.7  
6.25% 2040 Senior Notes [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount               35.7  
Gain (loss) on extinguishment of debt               $ 12.9  
AK Steel Holding Corporation [Member] | Unsecured Industrial Revenue Bonds [Member]                  
Extinguishment of Debt [Line Items]                  
Extinguishment of Debt, Amount $ 7.3