Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement	[1]									$ (113)	$ (114)	$ (216)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement										19,133	18,291	17,467
Reclassification adjustment impacting general and administrative expense on the income statement										4,365	4,297	4,170
Total reclassifications		$ (1,074)	$ (432)	$ (665)	$ (967)	$ (746)	$ (806)	$ (631)	$ (778)	(3,138)	(2,961)	(2,730)
Income tax (expense) benefit		(201)	(36)	(108)	(171)	(125)	(97)	(107)	(109)	(516)	(438)	(674)
Amounts reclassified from accumulated other comprehensive income, net of taxes		$ (873)	$ (396)	$ (557)	$ (796)	$ (621)	$ (709)	$ (524)	$ (669)	(2,622)	(2,523)	(2,056)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications										4	54	(147)
Income tax (expense) benefit										(1)	11	(41)
Amounts reclassified from accumulated other comprehensive income, net of taxes										5	43	(106)
Changes in Net Unrealized Gains (Losses) 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 net realized investment gains on the income statement										(55)	(32)	(228)
Income tax (expense) benefit										(12)	(7)	(80)
Amounts reclassified from accumulated other comprehensive income, net of taxes										(43)	(25)	(148)
Changes in Net Unrealized Gains (Losses) 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 net realized investment gains on the income statement										0	0	1
Income tax (expense) benefit										0	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes										0	0	1
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										21	35	32
Reclassification adjustment impacting general and administrative expense on the income statement										31	51	48
Total reclassifications										52	86	80
Income tax (expense) benefit										11	18	39
Amounts reclassified from accumulated other comprehensive income, net of taxes										41	68	41
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 net realized investment gains on the income statement										7	0	0
Income tax (expense) benefit										0	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes										$ 7	$ 0	$ 0

[1] Total other-than-temporary impairment (OTTI) losses were $(3) million, $(1) million and $(13) million for the years ended December 31, 2019, 2018 and 2017, respectively.  Of total OTTI, credit losses of $(4) million, $(1) million and $(14) million for the years ended December 31, 2019, 2018 and 2017, respectively, were recognized in net realized investment gains.  In addition, unrealized gains (losses) from other changes in total OTTI of $1 million, $0 million and $1 million for the years ended December 31, 2019, 2018 and 2017, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income.