Debt Schedule of Debt Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Principal amount of debt	$ 1,591.3	$ 2,015.6
Less unamortized discount	(10.6)	(16.8)
Revolving Credit Facility due 2022 [Member]
Debt Instrument [Line Items]
Principal amount of debt	0.0	0.0
Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Principal amount of debt	0.0	382.4
Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Principal amount of debt	465.1	500.0
Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Principal amount of debt	636.8	650.0
Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Principal amount of debt	$ 500.0	$ 500.0