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
Sep. 30, 2020
Dec. 31, 2019
Carrying Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value $ 4,309.8 $ 2,113.8
Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total long-term debt, fair value 4,465.4 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 3,815.5 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 3,975.2 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 90.2 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 400.0 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 $ 400.0 $ 0.0