Pension Plan and Other Postretirement Benefits (Pension and Other Postretirement Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 2
Interest cost	2	1
Expected return on plan assets	(2)	(1)
Amortization of prior service cost
Amortization of net loss		1
Net periodic benefit cost	3	3
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1	$ 1