Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Investment disclosure details
Fixed maturities, amortized cost	$ 60,313	$ 59,994
Equity securities, cost	436	414
Fixed maturities, at fair value	64,705	64,232
Equity securities, at fair value	595	559
Continuous unrealized loss position less than 12 months, fair value	3,319	2,237
Continuous unrealized loss position less than 12 months, gross unrealized losses	33	59
Continuous unrealized loss position 12 months or longer, fair value	321	460
Continuous unrealized loss position 12 months or longer, gross unrealized losses	27	34
Continuous unrealized loss position, total, fair value	3,640	2,697
Continuous unrealized loss position, total, gross unrealized losses	60	93
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	9
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	11
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	60,313	59,994
Gross unrealized gains	4,448	4,327
Gross unrealized losses	56	89
Fixed maturities, at fair value	64,705	64,232
Continuous unrealized loss position less than 12 months, fair value	3,229	2,136
Continuous unrealized loss position less than 12 months, gross unrealized losses	30	55
Continuous unrealized loss position 12 months or longer, fair value	312	453
Continuous unrealized loss position 12 months or longer, gross unrealized losses	26	34
Continuous unrealized loss position, total, fair value	3,541	2,589
Continuous unrealized loss position, total, gross unrealized losses	56	89
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	9
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	11
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,072	2,396
Gross unrealized gains	85	101
Fixed maturities, at fair value	2,157	2,497
Continuous unrealized loss position less than 12 months, fair value	919	356
Continuous unrealized loss position, total, fair value	919	356
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	36,150	36,211
Gross unrealized gains	2,816	2,816
Gross unrealized losses	15	5
Fixed maturities, at fair value	38,951	39,022
Continuous unrealized loss position less than 12 months, fair value	1,286	27
Continuous unrealized loss position less than 12 months, gross unrealized losses	12
Continuous unrealized loss position 12 months or longer, fair value	30	191
Continuous unrealized loss position 12 months or longer, gross unrealized losses	3	5
Continuous unrealized loss position, total, fair value	1,316	218
Continuous unrealized loss position, total, gross unrealized losses	15	5
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	7,720	6,820
Gross unrealized gains	585	513
Gross unrealized losses		1
Fixed maturities, at fair value	8,305	7,332
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,430	29,391
Gross unrealized gains	2,231	2,303
Gross unrealized losses	15	4
Fixed maturities, at fair value	30,646	31,690
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,250	2,228
Gross unrealized gains	85	91
Gross unrealized losses	1	1
Fixed maturities, at fair value	2,334	2,318
Continuous unrealized loss position less than 12 months, fair value	151	96
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	1
Continuous unrealized loss position 12 months or longer, fair value		2
Continuous unrealized loss position, total, fair value	151	98
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	3,075	3,288
Gross unrealized gains	250	249
Gross unrealized losses	7	22
Fixed maturities, at fair value	3,318	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	1	12
Continuous unrealized loss position 12 months or longer, fair value	145	155
Continuous unrealized loss position 12 months or longer, gross unrealized losses	6	10
Continuous unrealized loss position, total, fair value	215	517
Continuous unrealized loss position, total, gross unrealized losses	7	22
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	16,740	15,845
Gross unrealized gains	1,207	1,066
Gross unrealized losses	33	61
Fixed maturities, at fair value	17,914	16,850
Continuous unrealized loss position less than 12 months, fair value	803	1,295
Continuous unrealized loss position less than 12 months, gross unrealized losses	16	42
Continuous unrealized loss position 12 months or longer, fair value	137	105
Continuous unrealized loss position 12 months or longer, gross unrealized losses	17	19
Continuous unrealized loss position, total, fair value	940	1,400
Continuous unrealized loss position, total, gross unrealized losses	33	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	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	9
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	11
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	26	26
Gross unrealized gains	5	4
Fixed maturities, at fair value	31	30
Equity securities [Member]
Investment disclosure details
Equity securities, cost	436	414
Gross unrealized gains	163	149
Gross unrealized losses	4	4
Equity securities, at fair value	595	559
Continuous unrealized loss position less than 12 months, fair value	90	101
Continuous unrealized loss position less than 12 months, gross unrealized losses	3	4
Continuous unrealized loss position 12 months or longer, fair value	9	7
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1
Continuous unrealized loss position, total, fair value	99	108
Continuous unrealized loss position, total, gross unrealized losses	4	4
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	340	311
Gross unrealized gains	130	120
Gross unrealized losses	3	3
Equity securities, at fair value	467	428
Continuous unrealized loss position less than 12 months, fair value	56	64
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	3
Continuous unrealized loss position 12 months or longer, fair value	4
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1
Continuous unrealized loss position, total, fair value	60	64
Continuous unrealized loss position, total, gross unrealized losses	3	3
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	96	103
Gross unrealized gains	33	29
Gross unrealized losses	1	1
Equity securities, at fair value	128	131
Continuous unrealized loss position less than 12 months, fair value	34	37
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	1
Continuous unrealized loss position 12 months or longer, fair value	5	7
Continuous unrealized loss position, total, fair value	39	44
Continuous unrealized loss position, total, gross unrealized losses	$ 1	$ 1