Pension Plan and Other Postretirement Benefits (Pension and Other Postretirement Benefit Costs) (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]
Service cost	$ 2	$ 2	$ 8	$ 7
Interest cost	1	2	4	4
Expected return on plan assets	(2)	(1)	(6)	(4)
Amortization of prior service cost
Amortization of net loss	1		1	1
Net periodic benefit cost	$ 2	3	7	8
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 1	$ 2	$ 2