Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - Reclassifications - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement	[1]	$ (61)	$ (23)	$ (146)	$ (33)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement		4,806	3,856	13,125	11,330
Reclassification adjustment impacting general and administrative expense on the income statement		1,045	1,057	3,086	3,106
Total reclassifications		(320)	(947)	(1,870)	(2,751)
Income tax (expense) benefit		(27)	(231)	(365)	(680)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(293)	(716)	(1,505)	(2,071)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications		(36)	(4)	(95)	4
Income tax (expense) benefit		(12)	(2)	(33)	1
Amounts reclassified from accumulated other comprehensive income, net of taxes		(24)	(2)	(62)	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		(64)	(19)	(158)	(54)
Income tax (expense) benefit		(22)	(7)	(55)	(19)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(42)	(12)	(103)	(35)
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		1		1	12
Income tax (expense) benefit					4
Amounts reclassified from accumulated other comprehensive income, net of taxes		1		1	8
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		11	6	25	18
Reclassification adjustment impacting general and administrative expense on the income statement		16	9	37	28
Total reclassifications		27	15	62	46
Income tax (expense) benefit		10	5	22	16
Amounts reclassified from accumulated other comprehensive income, net of taxes		$ 17	$ 10	$ 40	$ 30

[1] Total other-than-temporary impairment (OTTI) losses were $(5) million and $(4) million for the three months ended September 30, 2017 and 2016, respectively, and $(11) million and $(36) million for the nine months ended September 30, 2017 and 2016, respectively. Of total OTTI, credit losses of $(5) million and $(4) million for the three months ended September 30, 2017 and 2016, respectively, and $(12) million and $(26) million for the nine months ended September 30, 2017 and 2016, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $0 million for each of the three months ended September 30, 2017 and 2016, and $1 million and $(10) million for the nine months ended September 30, 2017 and 2016, 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.