Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement									$ (2)	$ (113)	$ (114)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement									19,123	19,133	18,291
Reclassification adjustment impacting general and administrative expense on the income statement									4,509	4,365	4,297
Total reclassifications	$ (1,616)	$ (986)	$ 85	$ (720)	$ (1,074)	$ (432)	$ (665)	$ (967)	(3,237)	(3,138)	(2,961)
Income tax (expense) benefit	(306)	(159)	45	(120)	(201)	(36)	(108)	(171)	(540)	(516)	(438)
Amounts reclassified from accumulated other comprehensive income, net of taxes	$ (1,310)	$ (827)	$ 40	$ (600)	$ (873)	$ (396)	$ (557)	$ (796)	(2,697)	(2,622)	(2,523)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications									32	4	54
Income tax (expense) benefit									7	(1)	11
Amounts reclassified from accumulated other comprehensive income, net of taxes									25	5	43
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 net realized investment gains on the income statement									(52)	(55)	(32)
Income tax (expense) benefit									(11)	(12)	(7)
Amounts reclassified from accumulated other comprehensive income, net of taxes									(41)	(43)	(25)
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 net realized investment gains on the income statement									0	0	0
Income tax (expense) benefit									0	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes									0	0	0
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									35	21	35
Reclassification adjustment impacting general and administrative expense on the income statement									50	31	51
Total reclassifications									85	52	86
Income tax (expense) benefit									18	11	18
Amounts reclassified from accumulated other comprehensive income, net of taxes									67	41	68
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									(1)	7	0
Income tax (expense) benefit									0	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes									$ (1)	$ 7	$ 0