Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment disclosure details
Fixed maturities, amortized cost	$ 61,003	$ 59,994
Equity securities, cost	439	414
Fixed maturities, at fair value	65,873	64,232
Equity securities, at fair value	629	559
Continuous unrealized loss position less than 12 months, fair value	835	2,237
Continuous unrealized loss position less than 12 months, gross unrealized losses	10	59
Continuous unrealized loss position 12 months or longer, fair value	195	460
Continuous unrealized loss position 12 months or longer, gross unrealized losses	14	34
Continuous unrealized loss position, total, fair value	1,030	2,697
Continuous unrealized loss position, total, gross unrealized losses	24	93
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 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	61,003	59,994
Gross unrealized gains	4,892	4,327
Gross unrealized losses	22	89
Fixed maturities, at fair value	65,873	64,232
Continuous unrealized loss position less than 12 months, fair value	792	2,136
Continuous unrealized loss position less than 12 months, gross unrealized losses	8	55
Continuous unrealized loss position 12 months or longer, fair value	195	453
Continuous unrealized loss position 12 months or longer, gross unrealized losses	14	34
Continuous unrealized loss position, total, fair value	987	2,589
Continuous unrealized loss position, total, gross unrealized losses	22	89
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 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	2,088	2,396
Gross unrealized gains	83	101
Fixed maturities, at fair value	2,171	2,497
Continuous unrealized loss position less than 12 months, fair value	51	356
Continuous unrealized loss position, total, fair value	51	356
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	36,241	36,211
Gross unrealized gains	3,014	2,816
Gross unrealized losses	3	5
Fixed maturities, at fair value	39,252	39,022
Continuous unrealized loss position less than 12 months, fair value	228	27
Continuous unrealized loss position 12 months or longer, fair value	45	191
Continuous unrealized loss position 12 months or longer, gross unrealized losses	3	5
Continuous unrealized loss position, total, fair value	273	218
Continuous unrealized loss position, total, gross unrealized losses	3	5
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	8,199	6,820
Gross unrealized gains	599	513
Gross unrealized losses		1
Fixed maturities, at fair value	8,798	7,332
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,042	29,391
Gross unrealized gains	2,415	2,303
Gross unrealized losses	3	4
Fixed maturities, at fair value	30,454	31,690
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,281	2,228
Gross unrealized gains	85	91
Gross unrealized losses		1
Fixed maturities, at fair value	2,366	2,318
Continuous unrealized loss position less than 12 months, fair value	37	96
Continuous unrealized loss position less than 12 months, gross unrealized losses		1
Continuous unrealized loss position 12 months or longer, fair value		2
Continuous unrealized loss position, total, fair value	37	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	2,872	3,288
Gross unrealized gains	284	249
Gross unrealized losses	2	22
Fixed maturities, at fair value	3,154	3,515
Continuous unrealized loss position less than 12 months, fair value	8	362
Continuous unrealized loss position less than 12 months, gross unrealized losses		12
Continuous unrealized loss position 12 months or longer, fair value	37	155
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	10
Continuous unrealized loss position, total, fair value	45	517
Continuous unrealized loss position, total, gross unrealized losses	2	22
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	17,495	15,845
Gross unrealized gains	1,419	1,066
Gross unrealized losses	17	61
Fixed maturities, at fair value	18,897	16,850
Continuous unrealized loss position less than 12 months, fair value	468	1,295
Continuous unrealized loss position less than 12 months, gross unrealized losses	8	42
Continuous unrealized loss position 12 months or longer, fair value	113	105
Continuous unrealized loss position 12 months or longer, gross unrealized losses	9	19
Continuous unrealized loss position, total, fair value	581	1,400
Continuous unrealized loss position, total, gross unrealized losses	17	61
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 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	439	414
Gross unrealized gains	192	149
Gross unrealized losses	2	4
Equity securities, at fair value	629	559
Continuous unrealized loss position less than 12 months, fair value	43	101
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	4
Continuous unrealized loss position 12 months or longer, fair value		7
Continuous unrealized loss position, total, fair value	43	108
Continuous unrealized loss position, total, gross unrealized losses	2	4
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	343	311
Gross unrealized gains	155	120
Gross unrealized losses	2	3
Equity securities, at fair value	496	428
Continuous unrealized loss position less than 12 months, fair value	26	64
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	3
Continuous unrealized loss position, total, fair value	26	64
Continuous unrealized loss position, total, gross unrealized losses	2	3
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	96	103
Gross unrealized gains	37	29
Gross unrealized losses		1
Equity securities, at fair value	133	131
Continuous unrealized loss position less than 12 months, fair value	17	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	17	44
Continuous unrealized loss position, total, gross unrealized losses		$ 1