Pension Plan and Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Employer contributions				$ 12
Treasury stock acquired, shares	4,829,011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)		$ 2	$ 3	7	$ 8
Benefit obligation	$ 34	34		34		$ 42
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)			$ 1	2	$ 2
Benefit obligation	$ 17	$ 17		$ 17		$ 17
Non-Qualified Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Treasury stock acquired, shares				10,653		31,269