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Employee Benefit Plans - Schedule of Pre-Tax Amounts Recognized in Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Defined Pension Benefits    
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]    
Prior service cost $ 0 $ 0
Net actuarial gain (loss) (49) (66)
Accumulated other comprehensive (loss) income (49) (66)
Other Postretirement Benefits    
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]    
Prior service cost (2) (2)
Net actuarial gain (loss) 10 12
Accumulated other comprehensive (loss) income $ 8 $ 10