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Long-Term Debt (6.250% Senior Notes Due 2021) (Details) - Senior Notes [Member] - USD ($)
Mar. 31, 2018
Jan. 23, 2018
Dec. 31, 2017
Apr. 07, 2016
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%
[1] (2)The Senior Notes were redeemed on January 23, 2018.