Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains (losses) on the income statement	$ (37)	$ (23)	$ 48	$ (101)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement	4,886	5,230	14,782	14,493
Reclassification adjustment impacting general and administrative expense on the income statement	1,109	1,098	3,367	3,280
Total reclassifications	(986)	(432)	(1,621)	(2,064)
Income tax (expense) benefit	(159)	(36)	(234)	(315)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(827)	(396)	(1,387)	(1,749)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications	(3)	(2)	32	(5)
Income tax (expense) benefit	(1)	0	6	(1)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(2)	(2)	26	(4)
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	(23)	(14)	(31)	(43)
Income tax (expense) benefit	(5)	(3)	(7)	(9)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(18)	(11)	(24)	(34)
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	8	4	26	15
Reclassification adjustment impacting general and administrative expense on the income statement	12	8	37	23
Total reclassifications	20	12	63	38
Income tax (expense) benefit	4	3	13	8
Amounts reclassified from accumulated other comprehensive income, net of taxes	16	9	50	30
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