Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ (14)	$ (31)	$ (28)	$ (38)
Reclassifications, net of tax			(21)	(27)
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax			4	3
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	3	8	7	11
Reclassifications, net of tax	(11)	(23)	(21)	(27)
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)	(6)	(28)	(13)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Pension Benefits [Member] \| Pension Settlement Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax		(25)		(25)
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	3	6	4
Tax impact	(1)	(1)	(2)	(1)
Reclassifications, net of tax	2	2	4	3
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	$ 3	$ 3	4	5
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)

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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