Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains (losses) on the income statement (loss)	$ (61)	$ (13)	$ (105)	$ 85
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement (loss)	5,045	5,107	10,015	9,896
Reclassification adjustment impacting general and administrative expense on the income statement (loss)	1,174	1,121	2,337	2,258
Total reclassifications	(1,131)	85	(2,022)	(635)
Income tax (expense) benefit	(197)	45	(355)	(75)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(934)	40	(1,667)	(560)
Accumulated Other Comprehensive Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications	2	12	13	35
Income tax (expense) benefit	1	3	3	7
Amounts reclassified from accumulated other comprehensive income, net of taxes	1	9	10	28
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 (loss)	(24)	(10)	(38)	(8)
Income tax (expense) benefit	(5)	(2)	(8)	(2)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(19)	(8)	(30)	(6)
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 (loss)	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 (loss)	11	9	21	18
Reclassification adjustment impacting general and administrative expense on the income statement (loss)	15	13	30	25
Total reclassifications	26	22	51	43
Income tax (expense) benefit	6	5	11	9
Amounts reclassified from accumulated other comprehensive income, net of taxes	20	17	40	34
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 (loss)	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