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Long-Term Debt (6.375% Senior Notes Due 2023) (Details) - Senior Notes [Member] - USD ($)
Mar. 31, 2018
Jan. 23, 2018
Dec. 31, 2017
Apr. 01, 2015
Debt Instrument [Line Items]        
Face amount   $ 2,200,000,000.0    
Six Point Three Seven Five Percentage Senior Notes Due Two Thousand Twenty Three [Member]        
Debt Instrument [Line Items]        
Senior Notes [1] $ 0   $ 800,000,000  
Face amount       $ 800,000,000
Interest rate, stated percentage       6.375%
Senior Notes Call Premium   $ 44,000,000    
[1] (2)The Senior Notes were redeemed on January 23, 2018.