Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	$ 3,658.9	$ 2,827.8
Debt Instrument, Fair Value Disclosure	3,789.7	2,885.7
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	3,658.9	2,827.8
Long-term Debt, Fair Value	3,789.7	2,885.7
Senior Unsecured Notes Due 2021 [Member] [Domain]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	299.7	299.1
Debt Instrument, Fair Value Disclosure	297.5	298.4
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	299.7	299.1
Long-term Debt, Fair Value	297.5	298.4
Senior Unsecured Notes Due 2028 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	694.6	694.1
Debt Instrument, Fair Value Disclosure	689.2	734.4
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	694.6	694.1
Long-term Debt, Fair Value	689.2	734.4
Secured Debt Term A [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	0.0	438.5
Debt Instrument, Fair Value Disclosure	0.0	440.1
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	0.0	438.5
Long-term Debt, Fair Value	0.0	440.1
Senior Secured Notes Due 2026 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	298.1	297.8
Debt Instrument, Fair Value Disclosure	313.9	305.6
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	298.1	297.8
Long-term Debt, Fair Value	313.9	305.6
2018 Revolver [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	0.0	800.0
Debt Instrument, Fair Value Disclosure	0.0	800.0
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	0.0	800.0
Long-term Debt, Fair Value	0.0	800.0
Senior Unsecured Notes Due 2023 [Member] [Domain]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	298.8	298.3
Debt Instrument, Fair Value Disclosure	293.8	307.2
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	298.8	298.3
Long-term Debt, Fair Value	293.8	307.2
Secured Debt Term B [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	389.6	0.0
Debt Instrument, Fair Value Disclosure	395.0	0.0
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	389.6	0.0
Long-term Debt, Fair Value	395.0	0.0
Senior Secured Notes Due 2025
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	493.9	0.0
Debt Instrument, Fair Value Disclosure	521.2	0.0
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	493.9	0.0
Long-term Debt, Fair Value	521.2	0.0
Senior Secured Second Lien Notes Due 2025
Fair Value Assets And Liabilities Measured On Recurring Basis Financial Statement Captions [Line Items]
Long-term Debt	1,184.2	0.0
Debt Instrument, Fair Value Disclosure	1,279.1	0.0
Carrying amount and estimated fair value of long term debt
Long-term Debt, Carrying Amount	1,184.2	0.0
Long-term Debt, Fair Value	$ 1,279.1	$ 0.0