Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains (losses) on the income statement	$ (8)	$ (37)	$ (113)	$ 48
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement	5,464	4,886	15,479	14,782
Reclassification adjustment impacting general and administrative expense on the income statement	1,187	1,109	3,524	3,367
Total reclassifications	(786)	(986)	(2,808)	(1,621)
Income tax (expense) benefit	(124)	(159)	(479)	(234)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(662)	(827)	(2,329)	(1,387)
Accumulated Other Comprehensive Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications	14	(3)	27	32
Income tax (expense) benefit	3	(1)	6	6
Amounts reclassified from accumulated other comprehensive income, net of taxes	11	(2)	21	26
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 (losses) on the income statement	(12)	(23)	(50)	(31)
Income tax (expense) benefit	(2)	(5)	(10)	(7)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(10)	(18)	(40)	(24)
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 (losses) on the income statement	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes	0	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	10	8	31	26
Reclassification adjustment impacting general and administrative expense on the income statement	16	12	46	37
Total reclassifications	26	20	77	63
Income tax (expense) benefit	5	4	16	13
Amounts reclassified from accumulated other comprehensive income, net of taxes	21	16	61	50
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 (losses) on the income statement	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes	$ 0	$ 0	$ 0	$ 0