Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Investment disclosure details
Fixed maturities, amortized cost	$ 59,994	$ 60,170
Equity securities, cost	414	372
Fixed maturities, at fair value	64,232	62,820
Equity securities, at fair value	559	519
Continuous unrealized loss position less than 12 months, fair value	2,237	7,293
Continuous unrealized loss position less than 12 months, gross unrealized losses	59	187
Continuous unrealized loss position 12 months or longer, fair value	460	812
Continuous unrealized loss position 12 months or longer, gross unrealized losses	34	64
Continuous unrealized loss position, total, fair value	2,697	8,105
Continuous unrealized loss position, total, gross unrealized losses	93	251
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 3 months, 6 months or less	5
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	6
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	22
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	59,994	60,170
Gross unrealized gains	4,327	2,896
Gross unrealized losses	89	246
Fixed maturities, at fair value	64,232	62,820
Continuous unrealized loss position less than 12 months, fair value	2,136	7,245
Continuous unrealized loss position less than 12 months, gross unrealized losses	55	186
Continuous unrealized loss position 12 months or longer, fair value	453	760
Continuous unrealized loss position 12 months or longer, gross unrealized losses	34	60
Continuous unrealized loss position, total, fair value	2,589	8,005
Continuous unrealized loss position, total, gross unrealized losses	89	246
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 3 months, 6 months or less	5
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	6
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	22
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,396	1,914
Gross unrealized gains	101	94
Fixed maturities, at fair value	2,497	2,008
Continuous unrealized loss position less than 12 months, fair value	356	155
Continuous unrealized loss position, total, fair value	356	155
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	36,211	38,064
Gross unrealized gains	2,816	1,626
Gross unrealized losses	5	155
Fixed maturities, at fair value	39,022	39,535
Continuous unrealized loss position less than 12 months, fair value	27	5,364
Continuous unrealized loss position less than 12 months, gross unrealized losses		149
Continuous unrealized loss position 12 months or longer, fair value	191	139
Continuous unrealized loss position 12 months or longer, gross unrealized losses	5	6
Continuous unrealized loss position, total, fair value	218	5,503
Continuous unrealized loss position, total, gross unrealized losses	5	155
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	6,820	6,787
Gross unrealized gains	513	505
Gross unrealized losses	1	1
Fixed maturities, at fair value	7,332	7,291
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	29,391	31,277
Gross unrealized gains	2,303	1,121
Gross unrealized losses	4	154
Fixed maturities, at fair value	31,690	32,244
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,228	2,156
Gross unrealized gains	91	50
Gross unrealized losses	1	4
Fixed maturities, at fair value	2,318	2,202
Continuous unrealized loss position less than 12 months, fair value	96	419
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	4
Continuous unrealized loss position 12 months or longer, fair value	2	13
Continuous unrealized loss position, total, fair value	98	432
Continuous unrealized loss position, total, gross unrealized losses	1	4
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,288	3,952
Gross unrealized gains	249	248
Gross unrealized losses	22	36
Fixed maturities, at fair value	3,515	4,164
Continuous unrealized loss position less than 12 months, fair value	362	77
Continuous unrealized loss position less than 12 months, gross unrealized losses	12	1
Continuous unrealized loss position 12 months or longer, fair value	155	420
Continuous unrealized loss position 12 months or longer, gross unrealized losses	10	35
Continuous unrealized loss position, total, fair value	517	497
Continuous unrealized loss position, total, gross unrealized losses	22	36
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, total	2
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	15,845	14,051
Gross unrealized gains	1,066	876
Gross unrealized losses	61	51
Fixed maturities, at fair value	16,850	14,876
Continuous unrealized loss position less than 12 months, fair value	1,295	1,230
Continuous unrealized loss position less than 12 months, gross unrealized losses	42	32
Continuous unrealized loss position 12 months or longer, fair value	105	185
Continuous unrealized loss position 12 months or longer, gross unrealized losses	19	19
Continuous unrealized loss position, total, fair value	1,400	1,415
Continuous unrealized loss position, total, gross unrealized losses	61	51
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	5
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 6 months, 12 months or less	6
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	20
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	26	33
Gross unrealized gains	4	2
Fixed maturities, at fair value	30	35
Continuous unrealized loss position 12 months or longer, fair value		3
Continuous unrealized loss position, total, fair value		3
Equity securities [Member]
Investment disclosure details
Equity securities, cost	414	372
Gross unrealized gains	149	152
Gross unrealized losses	4	5
Equity securities, at fair value	559	519
Continuous unrealized loss position less than 12 months, fair value	101	48
Continuous unrealized loss position less than 12 months, gross unrealized losses	4	1
Continuous unrealized loss position 12 months or longer, fair value	7	52
Continuous unrealized loss position 12 months or longer, gross unrealized losses		4
Continuous unrealized loss position, total, fair value	108	100
Continuous unrealized loss position, total, gross unrealized losses	4	5
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	311	198
Gross unrealized gains	120	106
Gross unrealized losses	3
Equity securities, at fair value	428	304
Continuous unrealized loss position less than 12 months, fair value	64	3
Continuous unrealized loss position less than 12 months, gross unrealized losses	3
Continuous unrealized loss position 12 months or longer, fair value		3
Continuous unrealized loss position, total, fair value	64	6
Continuous unrealized loss position, total, gross unrealized losses	3
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	103	174
Gross unrealized gains	29	46
Gross unrealized losses	1	5
Equity securities, at fair value	131	215
Continuous unrealized loss position less than 12 months, fair value	37	45
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	1
Continuous unrealized loss position 12 months or longer, fair value	7	49
Continuous unrealized loss position 12 months or longer, gross unrealized losses		4
Continuous unrealized loss position, total, fair value	44	94
Continuous unrealized loss position, total, gross unrealized losses	$ 1	$ 5