Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains (losses) on the income statement	[1]	$ 11	$ (5)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement		4,296	4,094
Reclassification adjustment impacting general and administrative expense on the income statement		1,062	996
Total reclassifications		(778)	(760)
Income tax (expense) benefit		(109)	(143)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(669)	(617)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications		22	7
Income tax (expense) benefit		5	2
Amounts reclassified from accumulated other comprehensive income, net of taxes		17	5
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			(10)
Income tax (expense) benefit			(3)
Amounts reclassified from accumulated other comprehensive income, net of taxes			(7)
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	7
Reclassification adjustment impacting general and administrative expense on the income statement		13	10
Total reclassifications		22	17
Income tax (expense) benefit		5	5
Amounts reclassified from accumulated other comprehensive income, net of taxes		$ 17	$ 12

[1] Total other-than-temporary impairment (OTTI) gains (losses) were $0 and $(1) million for the three months ended March 31, 2018 and 2017, respectively. Of total OTTI, credit losses of $0 and $(2) million for the three months ended March 31, 2018 and 2017, respectively, were recognized in net realized investment gains (losses). In addition, unrealized gains from other changes in total OTTI of $0 and $1 million for the three months ended March 31, 2018 and 2017, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.