Long-Term Debt (5.500% Senior Notes Due 2020) (Details) - Senior Notes [Member] - USD ($)		Mar. 31, 2018	Jan. 23, 2018	Dec. 31, 2017	Apr. 07, 2016	Jul. 20, 2015
Debt Instrument [Line Items]
Face amount			$ 2,200,000,000.0
Six Point Two Five Zero Percentage Senior Notes Due Two Thousand Twenty One [Member]
Debt Instrument [Line Items]
Senior Notes	[1]	$ 0		$ 800,000,000
Face amount					$ 800,000,000
Senior Notes Call Premium			32,000,000
Interest rate, stated percentage					6.25%
Five Point Five Zero Zero Percentage Senior Notes Due Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Senior Notes	[1]	$ 0		$ 600,000,000
Face amount						$ 600,000,000
Senior Notes Call Premium			$ 17,000,000
Interest rate, stated percentage						5.50%

[1] (2)The Senior Notes were redeemed on January 23, 2018.