Quarterly report pursuant to Section 13 or 15(d)

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

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FAIR VALUE MEASUREMENTS - Carrying Value And Fair Value Of Financial Instruments Disclosure (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Carrying Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value $ 4,451.6 $ 2,113.8
Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 4,317.8 2,237.0
Senior Notes [Member] | Carrying Value [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 3,807.3 2,113.8
Senior Notes [Member] | Fair Value [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 3,681.8 2,237.0
Industrial Revenue Bonds [Member] | Carrying Value [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 94.3 0.0
Industrial Revenue Bonds [Member] | Fair Value [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 86.0 0.0
Line of Credit [Member] | Carrying Value [Member] | Fair Value, Inputs, Level 2 [Member] | Credit Facility    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 550.0 0.0
Line of Credit [Member] | Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Credit Facility    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value $ 550.0 $ 0.0