Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investment disclosure details
Fixed maturities, amortized cost	$ 60,578	$ 60,829
Equity securities, cost	446	462
Fixed maturities, at fair value	64,699	65,393
Equity securities, at fair value	694	645
Continuous unrealized loss position less than 12 months, fair value	4,008	2,606
Continuous unrealized loss position less than 12 months, gross unrealized losses	64	27
Continuous unrealized loss position 12 months or longer, fair value	150	172
Continuous unrealized loss position 12 months or longer, gross unrealized losses	10	11
Continuous unrealized loss position, total, fair value	4,158	2,778
Continuous unrealized loss position, total, gross unrealized losses	74	38
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 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	5
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	60,578	60,829
Gross unrealized gains	4,195	4,598
Gross unrealized losses	74	34
Fixed maturities, at fair value	64,699	65,393
Continuous unrealized loss position less than 12 months, fair value	3,966	2,553
Continuous unrealized loss position less than 12 months, gross unrealized losses	64	23
Continuous unrealized loss position 12 months or longer, fair value	150	172
Continuous unrealized loss position 12 months or longer, gross unrealized losses	10	11
Continuous unrealized loss position, total, fair value	4,116	2,725
Continuous unrealized loss position, total, gross unrealized losses	74	34
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 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	5
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,070	2,148
Gross unrealized gains	67	75
Gross unrealized losses	1	1
Fixed maturities, at fair value	2,136	2,222
Continuous unrealized loss position less than 12 months, fair value	310	589
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	1
Continuous unrealized loss position, total, fair value	310	589
Continuous unrealized loss position, total, gross unrealized losses	1	1
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	35,763	35,863
Gross unrealized gains	2,523	2,829
Gross unrealized losses	40	11
Fixed maturities, at fair value	38,246	38,681
Continuous unrealized loss position less than 12 months, fair value	1,928	611
Continuous unrealized loss position less than 12 months, gross unrealized losses	39	9
Continuous unrealized loss position 12 months or longer, fair value	46	45
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1	2
Continuous unrealized loss position, total, fair value	1,974	656
Continuous unrealized loss position, total, gross unrealized losses	40	11
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,059	8,458
Gross unrealized gains	589	567
Fixed maturities, at fair value	9,648	9,025
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	26,704	27,405
Gross unrealized gains	1,934	2,262
Gross unrealized losses	40	11
Fixed maturities, at fair value	28,598	29,656
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,121	2,185
Gross unrealized gains	68	72
Fixed maturities, at fair value	2,189	2,257
Continuous unrealized loss position less than 12 months, fair value	78	186
Continuous unrealized loss position 12 months or longer, fair value		2
Continuous unrealized loss position, total, fair value	78	188
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,532	2,744
Gross unrealized gains	246	255
Gross unrealized losses	2	2
Fixed maturities, at fair value	2,776	2,997
Continuous unrealized loss position less than 12 months, fair value	31	70
Continuous unrealized loss position 12 months or longer, fair value	23	36
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	2
Continuous unrealized loss position, total, fair value	54	106
Continuous unrealized loss position, total, gross unrealized losses	2	2
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	18,066	17,863
Gross unrealized gains	1,284	1,360
Gross unrealized losses	31	20
Fixed maturities, at fair value	19,319	19,203
Continuous unrealized loss position less than 12 months, fair value	1,619	1,097
Continuous unrealized loss position less than 12 months, gross unrealized losses	24	13
Continuous unrealized loss position 12 months or longer, fair value	81	89
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7	7
Continuous unrealized loss position, total, fair value	1,700	1,186
Continuous unrealized loss position, total, gross unrealized losses	31	20
Other fixed maturities [Member]
Investment disclosure details
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	1
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 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	5
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	26	26
Gross unrealized gains	7	7
Fixed maturities, at fair value	33	33
Equity securities [Member]
Investment disclosure details
Equity securities, cost	446	462
Gross unrealized gains	248	187
Gross unrealized losses		4
Equity securities, at fair value	694	645
Continuous unrealized loss position less than 12 months, fair value	42	53
Continuous unrealized loss position less than 12 months, gross unrealized losses		4
Continuous unrealized loss position, total, fair value	42	53
Continuous unrealized loss position, total, gross unrealized losses		4
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	352	366
Gross unrealized gains	209	148
Gross unrealized losses		4
Equity securities, at fair value	561	510
Continuous unrealized loss position less than 12 months, fair value	13	40
Continuous unrealized loss position less than 12 months, gross unrealized losses		4
Continuous unrealized loss position, total, fair value	13	40
Continuous unrealized loss position, total, gross unrealized losses		4
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	94	96
Gross unrealized gains	39	39
Equity securities, at fair value	133	135
Continuous unrealized loss position less than 12 months, fair value	29	13
Continuous unrealized loss position, total, fair value	$ 29	$ 13