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Debt and Financing - Schedule of Carrying Values and Estimated Fair Values of Debt Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Book Value    
Debt Instrument [Line Items]    
Long-term debt, fair value $ 1,548.4 $ 1,225.4
Fair Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 1,531.9 1,226.6
UST Loan | Book Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 667.4 349.2
UST Loan | Fair Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 627.3 322.0
Term Loan | Book Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 588.7 582.7
Term Loan | Fair Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 612.5 611.0
Lease financing obligations | Book Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 224.4 225.7
Lease financing obligations | Fair Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 224.1 225.8
Second A&R CDA | Book Value    
Debt Instrument [Line Items]    
Long-term debt, fair value 67.9 67.8
Second A&R CDA | Fair Value    
Debt Instrument [Line Items]    
Long-term debt, fair value $ 68.0 $ 67.8