Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	$ (10)	$ (4)
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	2	1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Benefits [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax impact	4	3
Reclassifications, net of tax	(10)	(4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Benefits [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	(14)	(7)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement Benefits Other Than Pensions (OPEB) [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	3	1
Tax impact	(1)
Reclassifications, net of tax	2	1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement Benefits Other Than Pensions (OPEB) [Member] | Prior-Service Credits (Costs) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	1	2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement Benefits Other Than Pensions (OPEB) [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ 2	$ (1)