Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investment disclosure details
Fixed maturities, amortized cost	$ 60,829	$ 59,994
Equity securities, cost	462	414
Fixed maturities, at fair value	65,393	64,232
Equity securities, at fair value	645	559
Continuous unrealized loss position less than 12 months, fair value	2,606	2,237
Continuous unrealized loss position less than 12 months, gross unrealized losses	27	59
Continuous unrealized loss position 12 months or longer, fair value	172	460
Continuous unrealized loss position 12 months or longer, gross unrealized losses	11	34
Continuous unrealized loss position, total, fair value	2,778	2,697
Continuous unrealized loss position, total, gross unrealized losses	38	93
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	60,829	59,994
Gross unrealized gains	4,598	4,327
Gross unrealized losses	34	89
Fixed maturities, at fair value	65,393	64,232
Continuous unrealized loss position less than 12 months, fair value	2,553	2,136
Continuous unrealized loss position less than 12 months, gross unrealized losses	23	55
Continuous unrealized loss position 12 months or longer, fair value	172	453
Continuous unrealized loss position 12 months or longer, gross unrealized losses	11	34
Continuous unrealized loss position, total, fair value	2,725	2,589
Continuous unrealized loss position, total, gross unrealized losses	34	89
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,148	2,396
Gross unrealized gains	75	101
Gross unrealized losses	1
Fixed maturities, at fair value	2,222	2,497
Continuous unrealized loss position less than 12 months, fair value	589	356
Continuous unrealized loss position less than 12 months, gross unrealized losses	1
Continuous unrealized loss position, total, fair value	589	356
Continuous unrealized loss position, total, gross unrealized losses	1
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	35,863	36,211
Gross unrealized gains	2,829	2,816
Gross unrealized losses	11	5
Fixed maturities, at fair value	38,681	39,022
Continuous unrealized loss position less than 12 months, fair value	611	27
Continuous unrealized loss position less than 12 months, gross unrealized losses	9
Continuous unrealized loss position 12 months or longer, fair value	45	191
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	5
Continuous unrealized loss position, total, fair value	656	218
Continuous unrealized loss position, total, gross unrealized losses	11	5
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	8,458	6,820
Gross unrealized gains	567	513
Gross unrealized losses		1
Fixed maturities, at fair value	9,025	7,332
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	27,405	29,391
Gross unrealized gains	2,262	2,303
Gross unrealized losses	11	4
Fixed maturities, at fair value	29,656	31,690
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,185	2,228
Gross unrealized gains	72	91
Gross unrealized losses		1
Fixed maturities, at fair value	2,257	2,318
Continuous unrealized loss position less than 12 months, fair value	186	96
Continuous unrealized loss position less than 12 months, gross unrealized losses		1
Continuous unrealized loss position 12 months or longer, fair value	2	2
Continuous unrealized loss position, total, fair value	188	98
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	2,744	3,288
Gross unrealized gains	255	249
Gross unrealized losses	2	22
Fixed maturities, at fair value	2,997	3,515
Continuous unrealized loss position less than 12 months, fair value	70	362
Continuous unrealized loss position less than 12 months, gross unrealized losses		12
Continuous unrealized loss position 12 months or longer, fair value	36	155
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	10
Continuous unrealized loss position, total, fair value	106	517
Continuous unrealized loss position, total, gross unrealized losses	2	22
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	17,863	15,845
Gross unrealized gains	1,360	1,066
Gross unrealized losses	20	61
Fixed maturities, at fair value	19,203	16,850
Continuous unrealized loss position less than 12 months, fair value	1,097	1,295
Continuous unrealized loss position less than 12 months, gross unrealized losses	13	42
Continuous unrealized loss position 12 months or longer, fair value	89	105
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7	19
Continuous unrealized loss position, total, fair value	1,186	1,400
Continuous unrealized loss position, total, gross unrealized losses	20	61
Other fixed maturities [Member]
Investment disclosure details
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	3
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	4
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	26	26
Gross unrealized gains	7	4
Fixed maturities, at fair value	33	30
Equity securities [Member]
Investment disclosure details
Equity securities, cost	462	414
Gross unrealized gains	187	149
Gross unrealized losses	4	4
Equity securities, at fair value	645	559
Continuous unrealized loss position less than 12 months, fair value	53	101
Continuous unrealized loss position less than 12 months, gross unrealized losses	4	4
Continuous unrealized loss position 12 months or longer, fair value		7
Continuous unrealized loss position, total, fair value	53	108
Continuous unrealized loss position, total, gross unrealized losses	4	4
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	366	311
Gross unrealized gains	148	120
Gross unrealized losses	4	3
Equity securities, at fair value	510	428
Continuous unrealized loss position less than 12 months, fair value	40	64
Continuous unrealized loss position less than 12 months, gross unrealized losses	4	3
Continuous unrealized loss position, total, fair value	40	64
Continuous unrealized loss position, total, gross unrealized losses	4	3
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	96	103
Gross unrealized gains	39	29
Gross unrealized losses		1
Equity securities, at fair value	135	131
Continuous unrealized loss position less than 12 months, fair value	13	37
Continuous unrealized loss position less than 12 months, gross unrealized losses		1
Continuous unrealized loss position 12 months or longer, fair value		7
Continuous unrealized loss position, total, fair value	13	44
Continuous unrealized loss position, total, gross unrealized losses		$ 1