Debt Schedule of Debt Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total principal amount of debt	$ 2,015.6	$ 2,507.1
Less unamortized discount	(16.8)	(22.2)
Revolving Credit Facility due 2019 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	0.0	430.0
Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	0.0	51.7
Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	382.4	397.6
Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	500.0	500.0
Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	650.0	650.0
Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	$ 500.0	$ 500.0