Debt (Schedule of Extinguishment of Debt - 2018 Tender Offers) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Feb. 28, 2018	Feb. 09, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Tender Offer						$ 750.0
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount				$ 194.1		1,130.6
Debt Instrument, Repurchase Amount				204.5		863.9
Senior Notes [Member] \| 8.50% Senior notes due 2019
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount				54.6	$ 145.8	62.0
Debt Instrument, Repurchase Amount				60.1		55.7
Senior Notes [Member] \| 6.875% Senior notes due 2020
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount				100.1	129.8	219.2
Debt Instrument, Repurchase Amount				105.1		181.5
Senior Notes [Member] \| 4.70% Senior notes due 2021
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount				39.4	$ 373.9	817.0
Debt Instrument, Repurchase Amount				$ 39.3		$ 609.0
Scenario, Forecast
Debt Instrument [Line Items]
Tender Offer	$ 985.0
Scenario, Forecast \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	650.4
Debt Instrument, Repurchase Amount	693.2
Scenario, Forecast \| Senior Notes [Member] \| 8.50% Senior notes due 2019
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	237.6	$ 55.0	$ 182.6
Debt Instrument, Repurchase Amount	256.8
Scenario, Forecast \| Senior Notes [Member] \| 6.875% Senior notes due 2020
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	256.6
Debt Instrument, Repurchase Amount	277.1
Scenario, Forecast \| Senior Notes [Member] \| 4.70% Senior notes due 2021
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount	156.2
Debt Instrument, Repurchase Amount	$ 159.3