Accumulated Other Comprehensive Income (Loss) (Components of Other Accumulated Comprehensive Income Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 7,584	$ 7,442
Cumulative effect of amended accounting standard		(14)
Other comprehensive (loss) before reclassifications	(492)	(734)
Amounts reclassified from accumulated other comprehensive loss	84	144
Other comprehensive (loss) income, net of tax	(408)	(590)	$ 1,050
Ending balance	7,943	7,584	7,442
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(4,647)	(4,043)	(5,093)
Other comprehensive (loss) income, net of tax	(408)	(590)	1,050
Ending balance	(5,055)	(4,647)	(4,043)
Unrealized Investment Gains (Losses)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	0	14
Cumulative effect of amended accounting standard		(14)
Other comprehensive (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive (loss) income, net of tax	0	0
Ending balance	0	0	14
Pension/Post-Retirement Plans Gains (Losses)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(2,953)	(2,892)
Cumulative effect of amended accounting standard		0
Other comprehensive (loss) before reclassifications	(643)	(205)
Amounts reclassified from accumulated other comprehensive loss	84	144
Other comprehensive (loss) income, net of tax	(559)	(61)
Ending balance	(3,512)	(2,953)	(2,892)
Foreign Currency Translation Adjustments
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1,694)	(1,165)
Cumulative effect of amended accounting standard		0
Other comprehensive (loss) before reclassifications	151	(529)
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive (loss) income, net of tax	151	(529)
Ending balance	$ (1,543)	$ (1,694)	$ (1,165)