Other Comprehensive Income (Loss) (Schedule of Components of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income, net of tax	$ 152	$ 256
Unrealized Investment Gains
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	14	19
Cumulative effect of new accounting principle in period of adoption			$ (14)
Other comprehensive (loss) income before reclassifications	0	(3)
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income, net of tax	0	(3)
Balance, ending of period	0	16
Pension/Postretirement Plans Gains (Losses)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(2,892)	(3,232)
Cumulative effect of new accounting principle in period of adoption			0
Other comprehensive (loss) income before reclassifications	(100)	(6)
Amounts reclassified from accumulated other comprehensive income	29	30
Other comprehensive income, net of tax	(71)	24
Balance, ending of period	(2,963)	(3,208)
Foreign Currency Translation Gains (Losses)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(1,165)	(1,880)
Cumulative effect of new accounting principle in period of adoption			0
Other comprehensive (loss) income before reclassifications	223	235
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income, net of tax	223	235
Balance, ending of period	(942)	(1,645)
AOCI Attributable to Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(4,043)	(5,093)
Cumulative effect of new accounting principle in period of adoption			$ (14)
Other comprehensive (loss) income before reclassifications	123	226
Amounts reclassified from accumulated other comprehensive income	29	30
Other comprehensive income, net of tax	152	256
Balance, ending of period	$ (3,905)	$ (4,837)