Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement	[1]	$ (36)	$ (80)	$ (25)	$ (85)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement		4,562	4,225	8,858	8,319
Reclassification adjustment impacting general and administrative expense on the income statement		1,113	1,045	2,175	2,041
Total reclassifications		(631)	(790)	(1,409)	(1,550)
Income tax (expense) benefit		(107)	(195)	(216)	(338)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(524)	(595)	(1,193)	(1,212)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications		9	(66)	31	(59)
Income tax (expense) benefit		1	(23)	6	(21)
Amounts reclassified from accumulated other comprehensive income, net of taxes		8	(43)	25	(38)
Changes in Net Unrealized Gains (Losses) 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		(12)	(84)	(12)	(94)
Income tax (expense) benefit		(3)	(30)	(3)	(33)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(9)	(54)	(9)	(61)
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		8	7	17	14
Reclassification adjustment impacting general and administrative expense on the income statement		13	11	26	21
Total reclassifications		21	18	43	35
Income tax (expense) benefit		4	7	9	12
Amounts reclassified from accumulated other comprehensive income, net of taxes		$ 17	$ 11	$ 34	$ 23

[1] Total other-than-temporary impairment (OTTI) losses were $(1) million and $(5) million for the three months ended June 30, 2018 and 2017, respectively, and $(1) million and $(6) million for the six months ended June 30, 2018 and 2017, respectively. Of total OTTI, credit losses of $(1) million and $(5) million for the three months ended June 30, 2018 and 2017, respectively, and $(1) million and $(7) million for the six months ended June 30, 2018 and 2017, 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 June 30, 2018 and 2017, respectively, and $0 million and $1 million for the six months ended June 30, 2018 and 2017, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income.