Pension and Other Postretirement Benefits - About our Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan cost	$ (4)	$ (1)	$ (8)	$ 4
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan cost	$ 1	$ 2	$ 3	$ 4