Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - Reclassifications - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting realized gains on the income statement	[1]									$ (216)	$ (68)	$ (3)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement										17,467	15,070	13,723
Reclassification adjustment impacting general and administrative expense on the income statement										4,170	4,154	4,094
Total reclassifications		$ (860)	$ (320)	$ (790)	$ (760)	$ (1,302)	$ (947)	$ (887)	$ (917)	(2,730)	(4,053)	(4,740)
Income tax (expense) benefit		(309)	(27)	(195)	(143)	(359)	(231)	(223)	(226)	(674)	(1,039)	(1,301)
Amounts reclassified from accumulated other comprehensive income, net of taxes		$ (551)	$ (293)	$ (595)	$ (617)	$ (943)	$ (716)	$ (664)	$ (691)	(2,056)	(3,014)	(3,439)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications										(147)	11	79
Income tax (expense) benefit										(41)	4	27
Amounts reclassified from accumulated other comprehensive income, net of taxes										(106)	7	52
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting realized gains on the income statement										(228)	(64)	(42)
Income tax (expense) benefit										(80)	(22)	(15)
Amounts reclassified from accumulated other comprehensive income, net of taxes										(148)	(42)	(27)
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting realized gains on the income statement										1	13	2
Income tax (expense) benefit											4
Amounts reclassified from accumulated other comprehensive income, net of taxes										1	9	2
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement										32	25	38
Reclassification adjustment impacting general and administrative expense on the income statement										48	37	55
Total reclassifications										80	62	93
Income tax (expense) benefit										39	22	33
Amounts reclassified from accumulated other comprehensive income, net of taxes										$ 41	$ 40	60
Net Unrealized Foreign Currency Translation [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting realized gains on the income statement												26
Income tax (expense) benefit												9
Amounts reclassified from accumulated other comprehensive income, net of taxes												$ 17

[1] Total other-than-temporary impairment (OTTI) losses were $(13) million, $(40) million and $(54) million for the years ended December 31, 2017, 2016 and 2015, respectively. Of total OTTI, credit losses of $(14) million, $(29) million and $(52) million for the years ended December 31, 2017, 2016 and 2015, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $1 million, $(11) million and $(2) million for the years ended December 31, 2017, 2016 and 2015, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statements of income.