Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement	$ 98	$ (53)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement	4,789	4,442
Reclassification adjustment impacting general and administrative expense on the income statement	1,137	1,057
Total reclassifications	(720)	(967)
Income tax (expense) benefit	(120)	(171)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(600)	(796)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications	23	(3)
Income tax (expense) benefit	4	0
Amounts reclassified from accumulated other comprehensive income, net of taxes	19	(3)
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	2	(16)
Income tax (expense) benefit	0	(3)
Amounts reclassified from accumulated other comprehensive income, net of taxes	2	(13)
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
Income tax (expense) benefit	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes	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	5
Reclassification adjustment impacting general and administrative expense on the income statement	12	8
Total reclassifications	21	13
Income tax (expense) benefit	4	3
Amounts reclassified from accumulated other comprehensive income, net of taxes	17	10
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
Income tax (expense) benefit	0	0
Amounts reclassified from accumulated other comprehensive income, net of taxes	$ 0	$ 0