Pension Plan and Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)	$ 2		$ 2	$ 2	$ 1
Employer contributions				8
Company's expected additional annual contribution	4			4
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)				5	$ 5
Benefit obligation	38			38		$ 42
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)	1
Benefit obligation	$ 18			$ 18		$ 17
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Common stock purchased under the terms of the Non-Qualified Plan presented as treasury stock		31,269				31,269