Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - Reclassifications - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Reclassification adjustment impacting net realized investment gains on the income statement	[1]	$ (23)	$ (29)	$ (101)	$ (54)
Reclassification adjustment impacting claims and claim adjustment expenses on the income statement		5,230	4,655	14,493	13,513
Reclassification adjustment impacting general and administrative expense on the income statement		1,098	1,059	3,280	3,234
Total reclassifications		(432)	(806)	(2,064)	(2,215)
Income tax (expense) benefit		(36)	(97)	(315)	(313)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(396)	(709)	(1,749)	(1,902)
Accumulated Other Comprehensive Income (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Total reclassifications		(2)	4	(5)	35
Income tax (expense) benefit		0	2	(1)	8
Amounts reclassified from accumulated other comprehensive income, net of taxes		(2)	2	(4)	27
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		(14)	(18)	(43)	(30)
Income tax (expense) benefit		(3)	(3)	(9)	(6)
Amounts reclassified from accumulated other comprehensive income, net of taxes		(11)	(15)	(34)	(24)
Changes in Net Unrealized Gains (Losses) 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		4	9	15	26
Reclassification adjustment impacting general and administrative expense on the income statement		8	13	23	39
Total reclassifications		12	22	38	65
Income tax (expense) benefit		3	5	8	14
Amounts reclassified from accumulated other comprehensive income, net of taxes		9	17	30	51
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

[1] Total other-than-temporary impairment (OTTI) losses were $0 million for each of the three months ended September 30, 2019 and 2018, and $(2) million and $(1) million for the nine months ended September 30, 2019 and September 30, 2018, respectively.  Of total OTTI, credit losses of $0 million for each of the three months ended September 30, 2019 and 2018, and $(2) million and $(1) million for the nine months ended September 30, 2019 and September 30, 2018, 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 and nine months ended September 30, 2019 and 2018 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.