Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement	$ (13)	$ (25)	$ 85	$ (78)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement	5,107	4,821	9,896	9,263
Reclassification adjustment impacting general and administrative expense on the income statement	1,121	1,125	2,258	2,182
Total reclassifications	85	(665)	(635)	(1,632)
Income tax (expense) benefit	45	(108)	(75)	(279)
Amounts reclassified from accumulated other comprehensive income, net of taxes	40	(557)	(560)	(1,353)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications	12	0	35	(3)
Income tax (expense) benefit	3	(1)	7	(1)
Amounts reclassified from accumulated other comprehensive income, net of taxes	9	1	28	(2)
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	(10)	(13)	(8)	(29)
Income tax (expense) benefit	(2)	(3)	(2)	(6)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(8)	(10)	(6)	(23)
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	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	9	6	18	11
Reclassification adjustment impacting general and administrative expense on the income statement	13	7	25	15
Total reclassifications	22	13	43	26
Income tax (expense) benefit	5	2	9	5
Amounts reclassified from accumulated other comprehensive income, net of taxes	17	11	34	21
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	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