Debt and Financing - Schedule of Carrying Values and Estimated Fair Values of Debt Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Book Value
Debt Instrument [Line Items]
Long-term debt, fair value	$ 1,523.7	$ 1,225.4
Fair Value
Debt Instrument [Line Items]
Long-term debt, fair value	1,511.1	1,226.6
UST Loan | Book Value
Debt Instrument [Line Items]
Long-term debt, fair value	643.9	349.2
UST Loan | Fair Value
Debt Instrument [Line Items]
Long-term debt, fair value	602.2	322.0
Term Loan | Book Value
Debt Instrument [Line Items]
Long-term debt, fair value	586.9	582.7
Term Loan | Fair Value
Debt Instrument [Line Items]
Long-term debt, fair value	615.8	611.0
Lease financing obligations | Book Value
Debt Instrument [Line Items]
Long-term debt, fair value	225.0	225.7
Lease financing obligations | Fair Value
Debt Instrument [Line Items]
Long-term debt, fair value	224.8	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	$ 68.3	$ 67.8