Investments (details) - Investment Information - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Investment disclosure details
Fixed maturities, amortized cost	$ 59,975		$ 59,975		$ 58,878
Gross unrealized gains	3,391		3,391		2,100
Gross unrealized losses	55		55		320
Fixed maturities, at fair value	63,311		63,311		60,658
Equity securities, cost	525		525		528
Gross unrealized gains	237		237		190
Gross unrealized losses	10		10		13
Equity securities, at fair value	752		752		705
Continuous unrealized loss position less than 12 months, fair value	1,561		1,561		11,221
Continuous unrealized loss position less than 12 months, gross unrealized losses	16		16		233
Continuous unrealized loss position 12 months or longer, fair value	873		873		1,008
Continuous unrealized loss position 12 months or longer, gross unrealized losses	49		49		100
Continuous unrealized loss position, total, fair value	2,434		2,434		12,229
Continuous unrealized loss position, total, gross unrealized losses	65		65		333
Unrealized losses for available-for-sale securities for which fair value is less than 80% of amortized cost, total	11		11
Impairment charges	$ 4	$ 6	$ 22	$ 9
Maxiumum [Member]
Investment disclosure details
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	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	1.00%		1.00%
Fixed maturities [Member]
Investment disclosure details
Gross realized gains			$ 46	40
Gross realized losses			8	3
Continuous unrealized loss position less than 12 months, fair value	$ 1,507		1,507		11,126
Continuous unrealized loss position less than 12 months, gross unrealized losses	13		13		224
Continuous unrealized loss position 12 months or longer, fair value	781		781		937
Continuous unrealized loss position 12 months or longer, gross unrealized losses	42		42		96
Continuous unrealized loss position, total, fair value	2,288		2,288		12,063
Continuous unrealized loss position, total, gross unrealized losses	55		55		320
Cumulative OTTI credit losses recognized for securities held	$ 88	$ 88	$ 88	$ 88
Fixed maturities [Member] | Maxiumum [Member]
Investment disclosure details
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	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	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	$ 2,038		$ 2,038		2,202
Gross unrealized gains	36		36		8
Gross unrealized losses	1		1		16
Fixed maturities, at fair value	2,073		2,073		2,194
Continuous unrealized loss position less than 12 months, fair value	37		37		1,820
Continuous unrealized loss position less than 12 months, gross unrealized losses					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	47		47		1,848
Continuous unrealized loss position, total, gross unrealized losses	1		1		16
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	31,534		31,534		30,027
Gross unrealized gains	2,099		2,099		1,393
Gross unrealized losses	1		1		9
Fixed maturities, at fair value	33,632		33,632		31,411
Continuous unrealized loss position less than 12 months, fair value	639		639		928
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		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	653		653		1,070
Continuous unrealized loss position, total, gross unrealized losses	1		1		9
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,816		13,816		12,744
Gross unrealized gains	954		954		577
Gross unrealized losses	1		1		3
Fixed maturities, at fair value	14,769		14,769		13,318
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	10,270		10,270		9,492
Gross unrealized gains	787		787		472
Gross unrealized losses					4
Fixed maturities, at fair value	11,057		11,057		9,960
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,958		1,958		1,978
Gross unrealized gains	124		124		97
Gross unrealized losses					2
Fixed maturities, at fair value	2,082		2,082		2,073
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	5,490		5,490		5,813
Gross unrealized gains	234		234		247
Fixed maturities, at fair value	5,724		5,724		6,060
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,745		1,745		1,829
Gross unrealized gains	56		56		45
Gross unrealized losses					1
Fixed maturities, at fair value	1,801		1,801		1,873
Continuous unrealized loss position less than 12 months, fair value	3		3		172
Continuous unrealized loss position less than 12 months, gross unrealized losses					1
Continuous unrealized loss position, total, fair value	3		3		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	1,702		1,702		1,863
Gross unrealized gains	132		132		124
Gross unrealized losses	1		1		6
Fixed maturities, at fair value	1,833		1,833		1,981
Continuous unrealized loss position less than 12 months, fair value	42		42		473
Continuous unrealized loss position less than 12 months, gross unrealized losses					4
Continuous unrealized loss position 12 months or longer, fair value	37		37		57
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1		2
Continuous unrealized loss position, total, fair value	79		79		530
Continuous unrealized loss position, total, gross unrealized losses	1		1		6
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	22,858		22,858		22,854
Gross unrealized gains	1,062		1,062		523
Gross unrealized losses	52		52		288
Fixed maturities, at fair value	23,868		23,868		23,089
Continuous unrealized loss position less than 12 months, fair value	779		779		7,725
Continuous unrealized loss position less than 12 months, gross unrealized losses	12		12		197
Continuous unrealized loss position 12 months or longer, fair value	720		720		710
Continuous unrealized loss position 12 months or longer, gross unrealized losses	40		40		91
Continuous unrealized loss position, total, fair value	1,499		1,499		8,435
Continuous unrealized loss position, total, gross unrealized losses	52		52		288
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	98		98		103
Gross unrealized gains	6		6		7
Fixed maturities, at fair value	104		104		110
Continuous unrealized loss position less than 12 months, fair value	7		7		8
Continuous unrealized loss position, total, fair value	7		7		8
Equity securities [Member]
Investment disclosure details
Gross realized gains			8	$ 5
Gross realized losses			2	$ 3
Continuous unrealized loss position less than 12 months, fair value	54		54		95
Continuous unrealized loss position less than 12 months, gross unrealized losses	3		3		9
Continuous unrealized loss position 12 months or longer, fair value	92		92		71
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7		7		4
Continuous unrealized loss position, total, fair value	146		146		166
Continuous unrealized loss position, total, gross unrealized losses	10		10		13
Public common stock [Member]
Investment disclosure details
Equity securities, cost	386		386		386
Gross unrealized gains	214		214		164
Gross unrealized losses	3		3		7
Equity securities, at fair value	597		597		543
Continuous unrealized loss position less than 12 months, fair value	24		24		48
Continuous unrealized loss position less than 12 months, gross unrealized losses	2		2		6
Continuous unrealized loss position 12 months or longer, fair value	34		34		33
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1		1
Continuous unrealized loss position, total, fair value	58		58		81
Continuous unrealized loss position, total, gross unrealized losses	3		3		7
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	139		139		142
Gross unrealized gains	23		23		26
Gross unrealized losses	7		7		6
Equity securities, at fair value	155		155		162
Continuous unrealized loss position less than 12 months, fair value	30		30		47
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		3
Continuous unrealized loss position 12 months or longer, fair value	58		58		38
Continuous unrealized loss position 12 months or longer, gross unrealized losses	6		6		3
Continuous unrealized loss position, total, fair value	88		88		85
Continuous unrealized loss position, total, gross unrealized losses	$ 7		$ 7		$ 6