Employee Benefit Plans - Postretirement (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (2)	$ (2)	$ (3)	$ (3)
Actuarial loss	1	1	1	1
Net benefit cost	$ (1)	$ (1)	$ (2)	$ (2)