Investments (details) - Investment Information - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Investment disclosure details
Fixed maturities, amortized cost	$ 60,149		$ 60,149		$ 58,878
Gross unrealized gains	2,939		2,939		2,100
Gross unrealized losses	52		52		320
Fixed maturities, at fair value	63,036		63,036		60,658
Equity securities, cost	510		510		528
Gross unrealized gains	242		242		190
Gross unrealized losses	8		8		13
Equity securities, at fair value	744		744		705
Continuous unrealized loss position less than 12 months, fair value	3,564		3,564		11,221
Continuous unrealized loss position less than 12 months, gross unrealized losses	27		27		233
Continuous unrealized loss position 12 months or longer, fair value	633		633		1,008
Continuous unrealized loss position 12 months or longer, gross unrealized losses	33		33		100
Continuous unrealized loss position, total, fair value	4,197		4,197		12,229
Continuous unrealized loss position, total, gross unrealized losses	60		60		333
Unrealized losses for available-for-sale securities for which fair value is less than 80% of amortized cost, total	$ 8		$ 8
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 pretax 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	$ 4	$ 14	$ 26	$ 23
Fixed maturities [Member]
Investment disclosure details
Gross realized gains			60	74
Gross realized losses			10	6
Continuous unrealized loss position less than 12 months, fair value	3,550		3,550		11,126
Continuous unrealized loss position less than 12 months, gross unrealized losses	26		26		224
Continuous unrealized loss position 12 months or longer, fair value	553		553		937
Continuous unrealized loss position 12 months or longer, gross unrealized losses	26		26		96
Continuous unrealized loss position, total, fair value	4,103		4,103		12,063
Continuous unrealized loss position, total, gross unrealized losses	52		52		320
Cumulative OTTI credit losses recognized for securities held	$ 86	$ 86	$ 86	$ 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 pretax 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	$ 2,046		$ 2,046		2,202
Gross unrealized gains	27		27		8
Gross unrealized losses	2		2		16
Fixed maturities, at fair value	2,071		2,071		2,194
Continuous unrealized loss position less than 12 months, fair value	146		146		1,820
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		15
Continuous unrealized loss position 12 months or longer, fair value	10		10		28
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1		1
Continuous unrealized loss position, total, fair value	156		156		1,848
Continuous unrealized loss position, total, gross unrealized losses	2		2		16
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	31,614		31,614		30,027
Gross unrealized gains	1,684		1,684		1,393
Gross unrealized losses	18		18		9
Fixed maturities, at fair value	33,280		33,280		31,411
Continuous unrealized loss position less than 12 months, fair value	2,255		2,255		928
Continuous unrealized loss position less than 12 months, gross unrealized losses	18		18		7
Continuous unrealized loss position 12 months or longer, fair value	14		14		142
Continuous unrealized loss position 12 months or longer, gross unrealized losses					2
Continuous unrealized loss position, total, fair value	2,269		2,269		1,070
Continuous unrealized loss position, total, gross unrealized losses	18		18		9
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,930		13,930		12,744
Gross unrealized gains	754		754		577
Gross unrealized losses	9		9		3
Fixed maturities, at fair value	14,675		14,675		13,318
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	10,668		10,668		9,492
Gross unrealized gains	608		608		472
Gross unrealized losses	6		6		4
Fixed maturities, at fair value	11,270		11,270		9,960
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,878		1,878		1,978
Gross unrealized gains	95		95		97
Gross unrealized losses	3		3		2
Fixed maturities, at fair value	1,970		1,970		2,073
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	5,138		5,138		5,813
Gross unrealized gains	227		227		247
Fixed maturities, at fair value	5,365		5,365		6,060
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,703		1,703		1,829
Gross unrealized gains	54		54		45
Gross unrealized losses					1
Fixed maturities, at fair value	1,757		1,757		1,873
Continuous unrealized loss position less than 12 months, fair value	31		31		172
Continuous unrealized loss position less than 12 months, gross unrealized losses					1
Continuous unrealized loss position, total, fair value	31		31		172
Continuous unrealized loss position, total, gross unrealized losses					1
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,651		1,651		1,863
Gross unrealized gains	128		128		124
Gross unrealized losses	1		1		6
Fixed maturities, at fair value	1,778		1,778		1,981
Continuous unrealized loss position less than 12 months, fair value	116		116		473
Continuous unrealized loss position less than 12 months, gross unrealized losses					4
Continuous unrealized loss position 12 months or longer, fair value	20		20		57
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1		2
Continuous unrealized loss position, total, fair value	136		136		530
Continuous unrealized loss position, total, gross unrealized losses	1		1		6
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	23,039		23,039		22,854
Gross unrealized gains	1,039		1,039		523
Gross unrealized losses	31		31		288
Fixed maturities, at fair value	24,047		24,047		23,089
Continuous unrealized loss position less than 12 months, fair value	1,002		1,002		7,725
Continuous unrealized loss position less than 12 months, gross unrealized losses	7		7		197
Continuous unrealized loss position 12 months or longer, fair value	502		502		710
Continuous unrealized loss position 12 months or longer, gross unrealized losses	24		24		91
Continuous unrealized loss position, total, fair value	1,504		1,504		8,435
Continuous unrealized loss position, total, gross unrealized losses	31		31		288
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	96		96		103
Gross unrealized gains	7		7		7
Fixed maturities, at fair value	103		103		110
Continuous unrealized loss position less than 12 months, fair value					8
Continuous unrealized loss position 12 months or longer, fair value	7		7
Continuous unrealized loss position, total, fair value	7		7		8
Equity securities [Member]
Investment disclosure details
Gross realized gains			12	$ 7
Gross realized losses			3	$ 4
Continuous unrealized loss position less than 12 months, fair value	14		14		95
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		9
Continuous unrealized loss position 12 months or longer, fair value	80		80		71
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7		7		4
Continuous unrealized loss position, total, fair value	94		94		166
Continuous unrealized loss position, total, gross unrealized losses	8		8		13
Public common stock [Member]
Investment disclosure details
Equity securities, cost	394		394		386
Gross unrealized gains	218		218		164
Gross unrealized losses	3		3		7
Equity securities, at fair value	609		609		543
Continuous unrealized loss position less than 12 months, fair value	9		9		48
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		6
Continuous unrealized loss position 12 months or longer, fair value	20		20		33
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2		2		1
Continuous unrealized loss position, total, fair value	29		29		81
Continuous unrealized loss position, total, gross unrealized losses	3		3		7
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	116		116		142
Gross unrealized gains	24		24		26
Gross unrealized losses	5		5		6
Equity securities, at fair value	135		135		162
Continuous unrealized loss position less than 12 months, fair value	5		5		47
Continuous unrealized loss position less than 12 months, gross unrealized losses					3
Continuous unrealized loss position 12 months or longer, fair value	60		60		38
Continuous unrealized loss position 12 months or longer, gross unrealized losses	5		5		3
Continuous unrealized loss position, total, fair value	65		65		85
Continuous unrealized loss position, total, gross unrealized losses	$ 5		$ 5		$ 6