Comprehensive Income (Loss) (Reclassification Out of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	$ (90)	$ (10)
Postretirement Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	5	2
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Pension Benefits [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	(120)	(14)
Tax impact	30	4
Reclassifications, net of tax	(90)	(10)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Pension Benefits [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	(17)	(14)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Pension Benefits [Member] | Pension Settlement Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	(103)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Postretirement Costs [Member] | Amortization Of Defined Benefit Cost Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	6	3
Tax impact	(1)	(1)
Reclassifications, net of tax	5	2
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Postretirement Costs [Member] | Prior-Service Credits (Costs) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	8	1
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Postretirement Costs [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, pretax	$ (2)	$ 2