Pension Plan and Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 5
Additional contributions for remainder of year		$ 7
Treasury stock acquired (in shares)	44,000,000	5,260,687		39,061,269
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)		$ 1	$ 1
Benefit obligation	$ 13	13		$ 13	$ 13
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (gain)		2	$ 3
Benefit obligation	$ 34	$ 31		$ 34	$ 34
Non-Qualified Supplemental Employee Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Treasury stock acquired (in shares)		5,115			10,653