Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Investment disclosure details
Fixed maturities, amortized cost	$ 60,777	$ 59,994
Equity securities, cost	419	414
Fixed maturities, at fair value	64,943	64,232
Equity securities, at fair value	583	559
Continuous unrealized loss position less than 12 months, fair value	3,657	2,237
Continuous unrealized loss position less than 12 months, gross unrealized losses	53	59
Continuous unrealized loss position 12 months or longer, fair value	345	460
Continuous unrealized loss position 12 months or longer, gross unrealized losses	27	34
Continuous unrealized loss position, total, fair value	4,002	2,697
Continuous unrealized loss position, total, gross unrealized losses	80	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 6 months, 12 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	12
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	15
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	60,777	59,994
Gross unrealized gains	4,243	4,327
Gross unrealized losses	77	89
Fixed maturities, at fair value	64,943	64,232
Continuous unrealized loss position less than 12 months, fair value	3,569	2,136
Continuous unrealized loss position less than 12 months, gross unrealized losses	50	55
Continuous unrealized loss position 12 months or longer, fair value	338	453
Continuous unrealized loss position 12 months or longer, gross unrealized losses	27	34
Continuous unrealized loss position, total, fair value	3,907	2,589
Continuous unrealized loss position, total, gross unrealized losses	77	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 6 months, 12 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	12
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	15
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,373	2,396
Gross unrealized gains	89	101
Fixed maturities, at fair value	2,462	2,497
Continuous unrealized loss position less than 12 months, fair value	1,072	356
Continuous unrealized loss position, total, fair value	1,072	356
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	36,333	36,211
Gross unrealized gains	2,708	2,816
Gross unrealized losses	26	5
Fixed maturities, at fair value	39,015	39,022
Continuous unrealized loss position less than 12 months, fair value	941	27
Continuous unrealized loss position less than 12 months, gross unrealized losses	22
Continuous unrealized loss position 12 months or longer, fair value	93	191
Continuous unrealized loss position 12 months or longer, gross unrealized losses	4	5
Continuous unrealized loss position, total, fair value	1,034	218
Continuous unrealized loss position, total, gross unrealized losses	26	5
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	7,362	6,820
Gross unrealized gains	571	513
Gross unrealized losses		1
Fixed maturities, at fair value	7,933	7,332
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	28,971	29,391
Gross unrealized gains	2,137	2,303
Gross unrealized losses	26	4
Fixed maturities, at fair value	31,082	31,690
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,314	2,228
Gross unrealized gains	78	91
Gross unrealized losses	1	1
Fixed maturities, at fair value	2,391	2,318
Continuous unrealized loss position less than 12 months, fair value	145	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	145	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	3,227	3,288
Gross unrealized gains	253	249
Gross unrealized losses	11	22
Fixed maturities, at fair value	3,469	3,515
Continuous unrealized loss position less than 12 months, fair value	207	362
Continuous unrealized loss position less than 12 months, gross unrealized losses	4	12
Continuous unrealized loss position 12 months or longer, fair value	135	155
Continuous unrealized loss position 12 months or longer, gross unrealized losses	7	10
Continuous unrealized loss position, total, fair value	342	517
Continuous unrealized loss position, total, gross unrealized losses	11	22
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, total	1
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	16,504	15,845
Gross unrealized gains	1,111	1,066
Gross unrealized losses	39	61
Fixed maturities, at fair value	17,576	16,850
Continuous unrealized loss position less than 12 months, fair value	1,204	1,295
Continuous unrealized loss position less than 12 months, gross unrealized losses	23	42
Continuous unrealized loss position 12 months or longer, fair value	110	105
Continuous unrealized loss position 12 months or longer, gross unrealized losses	16	19
Continuous unrealized loss position, total, fair value	1,314	1,400
Continuous unrealized loss position, total, gross unrealized losses	39	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 6 months, 12 months or less	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	12
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	14
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	26	26
Gross unrealized gains	4	4
Fixed maturities, at fair value	30	30
Equity securities [Member]
Investment disclosure details
Equity securities, cost	419	414
Gross unrealized gains	167	149
Gross unrealized losses	3	4
Equity securities, at fair value	583	559
Continuous unrealized loss position less than 12 months, fair value	88	101
Continuous unrealized loss position less than 12 months, gross unrealized losses	3	4
Continuous unrealized loss position 12 months or longer, fair value	7	7
Continuous unrealized loss position, total, fair value	95	108
Continuous unrealized loss position, total, gross unrealized losses	3	4
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	320	311
Gross unrealized gains	133	120
Gross unrealized losses	2	3
Equity securities, at fair value	451	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, total, fair value	56	64
Continuous unrealized loss position, total, gross unrealized losses	2	3
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	99	103
Gross unrealized gains	34	29
Gross unrealized losses	1	1
Equity securities, at fair value	132	131
Continuous unrealized loss position less than 12 months, fair value	32	37
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	1
Continuous unrealized loss position 12 months or longer, fair value	7	7
Continuous unrealized loss position, total, fair value	39	44
Continuous unrealized loss position, total, gross unrealized losses	$ 1	$ 1