Investments (details) - Investment Information - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investment disclosure details
Fixed maturities, amortized cost	$ 60,727		$ 60,727		$ 59,650
Gross unrealized gains	1,658		1,658		1,369
Gross unrealized losses	228		228		504
Fixed maturities, at fair value	62,157		62,157		60,515
Equity securities, cost	509		509		504
Gross unrealized gains	100		100		236
Gross unrealized losses	8		8		8
Equity securities, at fair value	601		601		732
Continuous unrealized loss position less than 12 months, fair value	11,681		11,681		18,310
Continuous unrealized loss position less than 12 months, gross unrealized losses	136		136		482
Continuous unrealized loss position 12 months or longer, fair value	2,496		2,496		561
Continuous unrealized loss position 12 months or longer, gross unrealized losses	100		100		30
Continuous unrealized loss position, total, fair value	14,177		14,177		18,871
Continuous unrealized loss position, total, gross unrealized losses	236		236		512
Unrealized losses for available-for-sale securities for which fair value is less than 80% of amortized cost, total	$ 1		$ 1
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of the combined fixed maturity and equity security portfolios on a pre-tax basis (less than)	1.00%		1.00%
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%		1.00%
Impairment charges	$ 5	$ 4	$ 12	$ 26
Fixed maturities [Member]
Investment disclosure details
Gross realized gains			28	60
Gross realized losses			6	10
Continuous unrealized loss position less than 12 months, fair value	11,642		11,642		18,263
Continuous unrealized loss position less than 12 months, gross unrealized losses	135		135		480
Continuous unrealized loss position 12 months or longer, fair value	2,430		2,430		492
Continuous unrealized loss position 12 months or longer, gross unrealized losses	93		93		24
Continuous unrealized loss position, total, fair value	14,072		14,072		18,755
Continuous unrealized loss position, total, gross unrealized losses	228		228		504
Cumulative OTTI credit losses recognized for securities held	$ 82	$ 86	$ 82	$ 86
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%	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%	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	$ 2,064		$ 2,064		2,031
Gross unrealized gains	9		9		9
Gross unrealized losses	5		5		5
Fixed maturities, at fair value	2,068		2,068		2,035
Continuous unrealized loss position less than 12 months, fair value	1,348		1,348		1,124
Continuous unrealized loss position less than 12 months, gross unrealized losses	4		4		5
Continuous unrealized loss position 12 months or longer, fair value	72		72
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1
Continuous unrealized loss position, total, fair value	1,420		1,420		1,124
Continuous unrealized loss position, total, gross unrealized losses	5		5		5
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	30,360		30,360		31,550
Gross unrealized gains	942		942		712
Gross unrealized losses	127		127		352
Fixed maturities, at fair value	31,175		31,175		31,910
Continuous unrealized loss position less than 12 months, fair value	4,131		4,131		9,781
Continuous unrealized loss position less than 12 months, gross unrealized losses	58		58		352
Continuous unrealized loss position 12 months or longer, fair value	1,598		1,598		12
Continuous unrealized loss position 12 months or longer, gross unrealized losses	69		69
Continuous unrealized loss position, total, fair value	5,729		5,729		9,793
Continuous unrealized loss position, total, gross unrealized losses	127		127		352
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,798		13,798		13,955
Gross unrealized gains	410		410		271
Gross unrealized losses	67		67		182
Fixed maturities, at fair value	14,141		14,141		14,044
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	11,341		11,341		10,910
Gross unrealized gains	324		324		215
Gross unrealized losses	51		51		147
Fixed maturities, at fair value	11,614		11,614		10,978
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,555		1,555		1,717
Gross unrealized gains	45		45		36
Gross unrealized losses	9		9		22
Fixed maturities, at fair value	1,591		1,591		1,731
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,666		3,666		4,968
Gross unrealized gains	163		163		190
Gross unrealized losses					1
Fixed maturities, at fair value	3,829		3,829		5,157
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,559		1,559		1,631
Gross unrealized gains	17		17		34
Gross unrealized losses	11		11		3
Fixed maturities, at fair value	1,565		1,565		1,662
Continuous unrealized loss position less than 12 months, fair value	538		538		360
Continuous unrealized loss position less than 12 months, gross unrealized losses	11		11		3
Continuous unrealized loss position, total, fair value	538		538		360
Continuous unrealized loss position, total, gross unrealized losses	11		11		3
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,082		2,082		1,614
Gross unrealized gains	97		97		100
Gross unrealized losses	6		6		6
Fixed maturities, at fair value	2,173		2,173		1,708
Continuous unrealized loss position less than 12 months, fair value	729		729		528
Continuous unrealized loss position less than 12 months, gross unrealized losses	5		5		5
Continuous unrealized loss position 12 months or longer, fair value	81		81		43
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1		1
Continuous unrealized loss position, total, fair value	810		810		571
Continuous unrealized loss position, total, gross unrealized losses	6		6		6
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	24,572		24,572		22,737
Gross unrealized gains	587		587		508
Gross unrealized losses	79		79		138
Fixed maturities, at fair value	25,080		25,080		23,107
Continuous unrealized loss position less than 12 months, fair value	4,896		4,896		6,470
Continuous unrealized loss position less than 12 months, gross unrealized losses	57		57		115
Continuous unrealized loss position 12 months or longer, fair value	679		679		437
Continuous unrealized loss position 12 months or longer, gross unrealized losses	22		22		23
Continuous unrealized loss position, total, fair value	5,575		5,575		6,907
Continuous unrealized loss position, total, gross unrealized losses	79		79		138
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	90		90		87
Gross unrealized gains	6		6		6
Fixed maturities, at fair value	96		96		93
Equity securities [Member]
Investment disclosure details
Gross realized gains			146	$ 12
Gross realized losses			1	$ 3
Continuous unrealized loss position less than 12 months, fair value	39		39		47
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		2
Continuous unrealized loss position 12 months or longer, fair value	66		66		69
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7		7		6
Continuous unrealized loss position, total, fair value	105		105		116
Continuous unrealized loss position, total, gross unrealized losses	8		8		8
Public common stock [Member]
Investment disclosure details
Equity securities, cost	392		392		390
Gross unrealized gains	80		80		216
Gross unrealized losses	2		2		3
Equity securities, at fair value	470		470		603
Continuous unrealized loss position less than 12 months, fair value	39		39		45
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		2
Continuous unrealized loss position 12 months or longer, fair value	6		6		10
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1		1
Continuous unrealized loss position, total, fair value	45		45		55
Continuous unrealized loss position, total, gross unrealized losses	2		2		3
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	117		117		114
Gross unrealized gains	20		20		20
Gross unrealized losses	6		6		5
Equity securities, at fair value	131		131		129
Continuous unrealized loss position less than 12 months, fair value					2
Continuous unrealized loss position 12 months or longer, fair value	60		60		59
Continuous unrealized loss position 12 months or longer, gross unrealized losses	6		6		5
Continuous unrealized loss position, total, fair value	60		60		61
Continuous unrealized loss position, total, gross unrealized losses	$ 6		$ 6		$ 5