Investments (details) - Investment Information - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investment disclosure details
Fixed maturities, amortized cost	$ 60,103		$ 60,103		$ 60,801
Fixed maturities, at fair value	61,933		61,933		63,474
Equity securities, cost	573		573		579
Equity securities, at fair value	828		828		899
Continuous unrealized loss position less than 12 months, fair value	14,289		14,289		2,744
Continuous unrealized loss position less than 12 months, gross unrealized losses	278		278		46
Continuous unrealized loss position 12 months or longer, fair value	934		934		3,536
Continuous unrealized loss position 12 months or longer, gross unrealized losses	46		46		78
Continuous unrealized loss position, total, fair value	15,223		15,223		6,280
Continuous unrealized loss position, total, gross unrealized losses	324		324		124
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	9		9
Impairment charges included in net realized investment gains in the consolidated statement of income	$ 6	$ 1	$ 9	$ 10
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
Fixed maturities, amortized cost	$ 60,103		$ 60,103		60,801
Gross unrealized gains	2,145		2,145		2,791
Gross unrealized losses	315		315		118
Fixed maturities, at fair value	61,933		61,933		63,474
Gross realized gains			40	23
Gross realized losses			3	4
Continuous unrealized loss position less than 12 months, fair value	14,183		14,183		2,619
Continuous unrealized loss position less than 12 months, gross unrealized losses	271		271		41
Continuous unrealized loss position 12 months or longer, fair value	855		855		3,493
Continuous unrealized loss position 12 months or longer, gross unrealized losses	44		44		77
Continuous unrealized loss position, total, fair value	15,038		15,038		6,112
Continuous unrealized loss position, total, gross unrealized losses	315		315		118
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	$ 88	$ 109	$ 88	$ 109
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%	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%	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	$ 2,025		$ 2,025		2,022
Gross unrealized gains	28		28		36
Gross unrealized losses	7		7		5
Fixed maturities, at fair value	2,046		2,046		2,053
Continuous unrealized loss position less than 12 months, fair value	507		507		180
Continuous unrealized loss position less than 12 months, gross unrealized losses	7		7		2
Continuous unrealized loss position 12 months or longer, fair value	24		24		125
Continuous unrealized loss position 12 months or longer, gross unrealized losses					3
Continuous unrealized loss position, total, fair value	531		531		305
Continuous unrealized loss position, total, gross unrealized losses	7		7		5
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	31,670		31,670		31,895
Gross unrealized gains	1,194		1,194		1,688
Gross unrealized losses	154		154		10
Fixed maturities, at fair value	32,710		32,710		33,573
Continuous unrealized loss position less than 12 months, fair value	7,737		7,737		173
Continuous unrealized loss position less than 12 months, gross unrealized losses	147		147		1
Continuous unrealized loss position 12 months or longer, fair value	135		135		797
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7		7		9
Continuous unrealized loss position, total, fair value	7,872		7,872		970
Continuous unrealized loss position, total, gross unrealized losses	154		154		10
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	6,966		6,966		7,229
Gross unrealized gains	306		306		332
Gross unrealized losses	1		1
Fixed maturities, at fair value	7,271		7,271		7,561
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	24,704		24,704		24,666
Gross unrealized gains	888		888		1,356
Gross unrealized losses	153		153		10
Fixed maturities, at fair value	25,439		25,439		26,012
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,049		2,049		2,320
Gross unrealized gains	52		52		48
Gross unrealized losses	1		1
Fixed maturities, at fair value	2,100		2,100		2,368
Continuous unrealized loss position less than 12 months, fair value	154		154		50
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1
Continuous unrealized loss position 12 months or longer, fair value					24
Continuous unrealized loss position, total, fair value	154		154		74
Continuous unrealized loss position, total, gross unrealized losses	1		1
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,905		1,905		2,052
Gross unrealized gains	147		147		165
Gross unrealized losses	6		6		4
Fixed maturities, at fair value	2,046		2,046		2,213
Continuous unrealized loss position less than 12 months, fair value	274		274		68
Continuous unrealized loss position less than 12 months, gross unrealized losses	2		2
Continuous unrealized loss position 12 months or longer, fair value	126		126		192
Continuous unrealized loss position 12 months or longer, gross unrealized losses	4		4		4
Continuous unrealized loss position, total, fair value	400		400		260
Continuous unrealized loss position, total, gross unrealized losses	6		6		4
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	22,341		22,341		22,390
Gross unrealized gains	714		714		844
Gross unrealized losses	147		147		99
Fixed maturities, at fair value	22,908		22,908		23,135
Continuous unrealized loss position less than 12 months, fair value	5,509		5,509		2,148
Continuous unrealized loss position less than 12 months, gross unrealized losses	114		114		38
Continuous unrealized loss position 12 months or longer, fair value	570		570		2,355
Continuous unrealized loss position 12 months or longer, gross unrealized losses	33		33		61
Continuous unrealized loss position, total, fair value	6,079		6,079		4,503
Continuous unrealized loss position, total, gross unrealized losses	147		147		99
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	113		113		122
Gross unrealized gains	10		10		10
Fixed maturities, at fair value	123		123		132
Continuous unrealized loss position less than 12 months, fair value	2		2
Continuous unrealized loss position, total, fair value	2		2
Equity securities [Member]
Investment disclosure details
Equity securities, cost	573		573		579
Gross unrealized gains	264		264		326
Gross unrealized losses	9		9		6
Equity securities, at fair value	828		828		899
Gross realized gains			5	$ 19
Gross realized losses			3	$ 3
Continuous unrealized loss position less than 12 months, fair value	106		106		125
Continuous unrealized loss position less than 12 months, gross unrealized losses	7		7		5
Continuous unrealized loss position 12 months or longer, fair value	79		79		43
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2		2		1
Continuous unrealized loss position, total, fair value	185		185		168
Continuous unrealized loss position, total, gross unrealized losses	9		9		6
Public common stock [Member]
Investment disclosure details
Equity securities, cost	410		410		400
Gross unrealized gains	235		235		295
Gross unrealized losses	6		6		4
Equity securities, at fair value	639		639		691
Continuous unrealized loss position less than 12 months, fair value	51		51		81
Continuous unrealized loss position less than 12 months, gross unrealized losses	6		6		4
Continuous unrealized loss position 12 months or longer, fair value	29		29		1
Continuous unrealized loss position, total, fair value	80		80		82
Continuous unrealized loss position, total, gross unrealized losses	6		6		4
Non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	163		163		179
Gross unrealized gains	29		29		31
Gross unrealized losses	3		3		2
Equity securities, at fair value	189		189		208
Continuous unrealized loss position less than 12 months, fair value	55		55		44
Continuous unrealized loss position less than 12 months, gross unrealized losses	1		1		1
Continuous unrealized loss position 12 months or longer, fair value	50		50		42
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2		2		1
Continuous unrealized loss position, total, fair value	105		105		86
Continuous unrealized loss position, total, gross unrealized losses	$ 3		$ 3		$ 2