Investments (Details) - Investment Information, Available for Sale Securities - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investment disclosure details
Fixed maturities, amortized cost	$ 62,093		$ 61,316
Gross unrealized gains	859		1,556
Gross unrealized losses	686		178
Fixed maturities, at fair value	62,266		62,694
Equity securities, available for sale, cost			440
Gross gains			20
Gross losses			7
Equity securities, available for sale, at fair value			453
Continuous unrealized loss position, less than 12 months, fair value			9,153
Continuous unrealized loss position, less than 12 months, gross unrealized losses			68
Continuous unrealized loss position, 12 months or longer, fair value			5,870
Continuous unrealized loss position, 12 months or longer, gross unrealized losses			117
Continuous unrealized loss position, total, fair value			15,023
Continuous unrealized loss position, total, gross unrealized losses			185
Impairment charges	0	$ 2
Fixed maturities [Member]
Investment disclosure details
Gross realized gains	6	7
Gross realized losses	6	2
Continuous unrealized loss position, less than 12 months, fair value	23,876		9,132
Continuous unrealized loss position, less than 12 months, gross unrealized losses	442		68
Continuous unrealized loss position, 12 months or longer, fair value	5,589		5,780
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	244		110
Continuous unrealized loss position, total, fair value	29,465		14,912
Continuous unrealized loss position, total, gross unrealized losses	686		178
Cumulative OTTI credit losses recognized for securities held	$ 75	$ 83
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of the fixed maturity portfolio on a pre-tax basis (less than)	1.00%	1.00%
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%	1.00%
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	$ 2,051		2,080
Gross unrealized gains	2		4
Gross unrealized losses	19		8
Fixed maturities, at fair value	2,034		2,076
Continuous unrealized loss position, less than 12 months, fair value	1,332		1,150
Continuous unrealized loss position, less than 12 months, gross unrealized losses	15		5
Continuous unrealized loss position, 12 months or longer, fair value	549		470
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	4		3
Continuous unrealized loss position, total, fair value	1,881		1,620
Continuous unrealized loss position, total, gross unrealized losses	19		8
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	29,134		29,996
Gross unrealized gains	536		968
Gross unrealized losses	230		49
Fixed maturities, at fair value	29,440		30,915
Continuous unrealized loss position, less than 12 months, fair value	5,823		505
Continuous unrealized loss position, less than 12 months, gross unrealized losses	94		2
Continuous unrealized loss position, 12 months or longer, fair value	2,863		2,959
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	136		47
Continuous unrealized loss position, total, fair value	8,686		3,464
Continuous unrealized loss position, total, gross unrealized losses	230		49
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,593		13,488
Gross unrealized gains	215		444
Gross unrealized losses	127		26
Fixed maturities, at fair value	13,681		13,906
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	10,512		11,307
Gross unrealized gains	177		338
Gross unrealized losses	91		19
Fixed maturities, at fair value	10,598		11,626
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,391		1,443
Gross unrealized gains	21		44
Gross unrealized losses	12		3
Fixed maturities, at fair value	1,400		1,484
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,638		3,758
Gross unrealized gains	123		142
Gross unrealized losses			1
Fixed maturities, at fair value	3,761		3,899
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,326		1,505
Gross unrealized gains	9		14
Gross unrealized losses	8		10
Fixed maturities, at fair value	1,327		1,509
Continuous unrealized loss position, less than 12 months, fair value	431		394
Continuous unrealized loss position, less than 12 months, gross unrealized losses	7		6
Continuous unrealized loss position, 12 months or longer, fair value	32		111
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1		4
Continuous unrealized loss position, total, fair value	463		505
Continuous unrealized loss position, total, gross unrealized losses	8		10
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,495		2,334
Gross unrealized gains	76		87
Gross unrealized losses	40		11
Fixed maturities, at fair value	2,531		2,410
Continuous unrealized loss position, less than 12 months, fair value	1,474		1,021
Continuous unrealized loss position, less than 12 months, gross unrealized losses	32		7
Continuous unrealized loss position, 12 months or longer, fair value	235		250
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	8		4
Continuous unrealized loss position, total, fair value	1,709		1,271
Continuous unrealized loss position, total, gross unrealized losses	40		11
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	27,000		25,311
Gross unrealized gains	231		478
Gross unrealized losses	389		100
Fixed maturities, at fair value	26,842		25,689
Continuous unrealized loss position, less than 12 months, fair value	14,816		6,062
Continuous unrealized loss position, less than 12 months, gross unrealized losses	294		48
Continuous unrealized loss position, 12 months or longer, fair value	1,910		1,990
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	95		52
Continuous unrealized loss position, total, fair value	16,726		8,052
Continuous unrealized loss position, total, gross unrealized losses	389		100
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	87		90
Gross unrealized gains	5		5
Fixed maturities, at fair value	$ 92		95
Equity securities [Member]
Investment disclosure details
Gross realized gains		$ 6
Gross realized losses		$ 1
Continuous unrealized loss position, less than 12 months, fair value			21
Continuous unrealized loss position, 12 months or longer, fair value			90
Continuous unrealized loss position, 12 months or longer, gross unrealized losses			7
Continuous unrealized loss position, total, fair value			111
Continuous unrealized loss position, total, gross unrealized losses			7
Public common stock [Member]
Investment disclosure details
Equity securities, available for sale, cost			332
Gross gains			8
Gross losses			1
Equity securities, available for sale, at fair value			339
Continuous unrealized loss position, less than 12 months, fair value			18
Continuous unrealized loss position, 12 months or longer, fair value			34
Continuous unrealized loss position, 12 months or longer, gross unrealized losses			1
Continuous unrealized loss position, total, fair value			52
Continuous unrealized loss position, total, gross unrealized losses			1
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, available for sale, cost			108
Gross gains			12
Gross losses			6
Equity securities, available for sale, at fair value			114
Continuous unrealized loss position, less than 12 months, fair value			3
Continuous unrealized loss position, 12 months or longer, fair value			56
Continuous unrealized loss position, 12 months or longer, gross unrealized losses			6
Continuous unrealized loss position, total, fair value			59
Continuous unrealized loss position, total, gross unrealized losses			$ 6