Pension and Other Postretirement Benefits - About our Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan cost	$ (3)	$ (3)	$ (11)	$ 1
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan cost	$ 2	$ 2	$ 5	$ 6