Investments (details) - Investment Information - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Investment disclosure details
Fixed maturities, amortized cost	$ 59,490		$ 58,878
Gross unrealized gains	2,636		2,100
Gross unrealized losses	141		320
Fixed maturities, at fair value	61,985		60,658
Equity securities, cost	528		528
Gross unrealized gains	198		190
Gross unrealized losses	16		13
Equity securities, at fair value	710		705
Continuous unrealized loss position less than 12 months, fair value	3,090		11,221
Continuous unrealized loss position less than 12 months, gross unrealized losses	82		233
Continuous unrealized loss position 12 months or longer, fair value	1,258		1,008
Continuous unrealized loss position 12 months or longer, gross unrealized losses	75		100
Continuous unrealized loss position, total, fair value	4,348		12,229
Continuous unrealized loss position, total, gross unrealized losses	157		333
Unrealized losses for available-for-sale securities for which fair value is less than 80% of amortized cost, total	50
Impairment charges	$ 18	$ 3
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%
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%
Fixed maturities [Member]
Investment disclosure details
Gross realized gains	$ 23	19
Gross realized losses	7	1
Continuous unrealized loss position less than 12 months, fair value	3,007		11,126
Continuous unrealized loss position less than 12 months, gross unrealized losses	73		224
Continuous unrealized loss position 12 months or longer, fair value	1,165		937
Continuous unrealized loss position 12 months or longer, gross unrealized losses	68		96
Continuous unrealized loss position, total, fair value	4,172		12,063
Continuous unrealized loss position, total, gross unrealized losses	141		320
Cumulative OTTI credit losses recognized for securities held	$ 92	$ 94
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 for fixed maturities held, as a percentage of the fixed maturity portfolio on a pretax basis	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 for fixed maturities held, as a percentage of shareholders' equity on an after-tax basis	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	$ 2,128		2,202
Gross unrealized gains	27		8
Gross unrealized losses	1		16
Fixed maturities, at fair value	2,154		2,194
Continuous unrealized loss position less than 12 months, fair value	171		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		28
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		1
Continuous unrealized loss position, total, fair value	181		1,848
Continuous unrealized loss position, total, gross unrealized losses	1		16
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	30,740		30,027
Gross unrealized gains	1,586		1,393
Gross unrealized losses	4		9
Fixed maturities, at fair value	32,322		31,411
Continuous unrealized loss position less than 12 months, fair value	681		928
Continuous unrealized loss position less than 12 months, gross unrealized losses	3		7
Continuous unrealized loss position 12 months or longer, fair value	141		142
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		2
Continuous unrealized loss position, total, fair value	822		1,070
Continuous unrealized loss position, total, gross unrealized losses	4		9
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	13,242		12,744
Gross unrealized gains	690		577
Gross unrealized losses	1		3
Fixed maturities, at fair value	13,931		13,318
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,914		9,492
Gross unrealized gains	563		472
Gross unrealized losses	3		4
Fixed maturities, at fair value	10,474		9,960
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,944		1,978
Gross unrealized gains	102		97
Gross unrealized losses			2
Fixed maturities, at fair value	2,046		2,073
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	5,640		5,813
Gross unrealized gains	231		247
Fixed maturities, at fair value	5,871		6,060
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,794		1,829
Gross unrealized gains	52		45
Gross unrealized losses			1
Fixed maturities, at fair value	1,846		1,873
Continuous unrealized loss position less than 12 months, fair value	25		172
Continuous unrealized loss position less than 12 months, gross unrealized losses			1
Continuous unrealized loss position, total, fair value	25		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	1,799		1,863
Gross unrealized gains	131		124
Gross unrealized losses	1		6
Fixed maturities, at fair value	1,929		1,981
Continuous unrealized loss position less than 12 months, fair value	98		473
Continuous unrealized loss position less than 12 months, gross unrealized losses			4
Continuous unrealized loss position 12 months or longer, fair value	73		57
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1		2
Continuous unrealized loss position, total, fair value	171		530
Continuous unrealized loss position, total, gross unrealized losses	1		6
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	22,921		22,854
Gross unrealized gains	834		523
Gross unrealized losses	135		288
Fixed maturities, at fair value	23,620		23,089
Continuous unrealized loss position less than 12 months, fair value	2,010		7,725
Continuous unrealized loss position less than 12 months, gross unrealized losses	70		197
Continuous unrealized loss position 12 months or longer, fair value	941		710
Continuous unrealized loss position 12 months or longer, gross unrealized losses	65		91
Continuous unrealized loss position, total, fair value	2,951		8,435
Continuous unrealized loss position, total, gross unrealized losses	135		288
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	108		103
Gross unrealized gains	6		7
Fixed maturities, at fair value	114		110
Continuous unrealized loss position less than 12 months, fair value	22		8
Continuous unrealized loss position, total, fair value	22		8
Equity securities [Member]
Investment disclosure details
Gross realized gains	3	$ 2
Gross realized losses	2	$ 0
Continuous unrealized loss position less than 12 months, fair value	83		95
Continuous unrealized loss position less than 12 months, gross unrealized losses	9		9
Continuous unrealized loss position 12 months or longer, fair value	93		71
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7		4
Continuous unrealized loss position, total, fair value	176		166
Continuous unrealized loss position, total, gross unrealized losses	16		13
Public common stock [Member]
Investment disclosure details
Equity securities, cost	383		386
Gross unrealized gains	174		164
Gross unrealized losses	8		7
Equity securities, at fair value	549		543
Continuous unrealized loss position less than 12 months, fair value	54		48
Continuous unrealized loss position less than 12 months, gross unrealized losses	8		6
Continuous unrealized loss position 12 months or longer, fair value	34		33
Continuous unrealized loss position 12 months or longer, gross unrealized losses			1
Continuous unrealized loss position, total, fair value	88		81
Continuous unrealized loss position, total, gross unrealized losses	8		7
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	145		142
Gross unrealized gains	24		26
Gross unrealized losses	8		6
Equity securities, at fair value	161		162
Continuous unrealized loss position less than 12 months, fair value	29		47
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		3
Continuous unrealized loss position 12 months or longer, fair value	59		38
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7		3
Continuous unrealized loss position, total, fair value	88		85
Continuous unrealized loss position, total, gross unrealized losses	$ 8		$ 6