Pension Plan and Other Postretirement Benefits (Pension and Other Postretirement Benefit Costs) (Details) - 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]
Service cost	$ 3	$ 3
Interest cost	1	1
Expected return on plan assets	(2)	(2)
Amortization of prior service cost
Net periodic benefit cost	2	$ 2	$ 2	$ 1
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			6	5
Interest cost			3	2
Expected return on plan assets			(4)	(3)
Amortization of prior service cost
Amortization of net loss				1
Net periodic benefit cost			$ 5	$ 5
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1