Employee Benefit Plans - Postretirement (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (1)	$ (1)
Net benefit cost	$ (1)	$ (1)