Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment disclosure details
Fixed maturities, amortized cost	$ 60,494	$ 60,829
Fixed maturities, at fair value	62,843	65,393
Equity securities, cost	463	462
Equity securities, at fair value	701	645
Continuous unrealized loss position less than 12 months, fair value	11,753	2,606
Continuous unrealized loss position less than 12 months, gross unrealized losses	551	27
Continuous unrealized loss position 12 months or longer, fair value	119	172
Continuous unrealized loss position 12 months or longer, gross unrealized losses	13	11
Continuous unrealized loss position, total, fair value	11,872	2,778
Continuous unrealized loss position, total, gross unrealized losses	564	38
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	4
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	6
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	60,494	60,829
Gross unrealized gains	2,911	4,598
Gross unrealized losses	562	34
Fixed maturities, at fair value	62,843	65,393
Continuous unrealized loss position less than 12 months, fair value	11,669	2,553
Continuous unrealized loss position less than 12 months, gross unrealized losses	549	23
Continuous unrealized loss position 12 months or longer, fair value	119	172
Continuous unrealized loss position 12 months or longer, gross unrealized losses	13	11
Continuous unrealized loss position, total, fair value	11,788	2,725
Continuous unrealized loss position, total, gross unrealized losses	562	34
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	4
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	6
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,058	2,148
Gross unrealized gains	53	75
Gross unrealized losses	8	1
Fixed maturities, at fair value	2,103	2,222
Continuous unrealized loss position less than 12 months, fair value	760	589
Continuous unrealized loss position less than 12 months, gross unrealized losses	8	1
Continuous unrealized loss position, total, fair value	760	589
Continuous unrealized loss position, total, gross unrealized losses	8	1
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	35,619	35,863
Gross unrealized gains	1,753	2,829
Gross unrealized losses	304	11
Fixed maturities, at fair value	37,068	38,681
Continuous unrealized loss position less than 12 months, fair value	4,585	611
Continuous unrealized loss position less than 12 months, gross unrealized losses	303	9
Continuous unrealized loss position 12 months or longer, fair value	29	45
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1	2
Continuous unrealized loss position, total, fair value	4,614	656
Continuous unrealized loss position, total, gross unrealized losses	304	11
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,395	8,458
Gross unrealized gains	538	567
Gross unrealized losses	1
Fixed maturities, at fair value	9,932	9,025
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	26,224	27,405
Gross unrealized gains	1,215	2,262
Gross unrealized losses	303	11
Fixed maturities, at fair value	27,136	29,656
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,039	2,185
Gross unrealized gains	44	72
Gross unrealized losses	4
Fixed maturities, at fair value	2,079	2,257
Continuous unrealized loss position less than 12 months, fair value	551	186
Continuous unrealized loss position less than 12 months, gross unrealized losses	4
Continuous unrealized loss position 12 months or longer, fair value		2
Continuous unrealized loss position, total, fair value	551	188
Continuous unrealized loss position, total, gross unrealized losses	4
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,410	2,744
Gross unrealized gains	197	255
Gross unrealized losses	12	2
Fixed maturities, at fair value	2,595	2,997
Continuous unrealized loss position less than 12 months, fair value	425	70
Continuous unrealized loss position less than 12 months, gross unrealized losses	10
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	448	106
Continuous unrealized loss position, total, gross unrealized losses	12	2
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	18,342	17,863
Gross unrealized gains	858	1,360
Gross unrealized losses	234	20
Fixed maturities, at fair value	18,966	19,203
Continuous unrealized loss position less than 12 months, fair value	5,348	1,097
Continuous unrealized loss position less than 12 months, gross unrealized losses	224	13
Continuous unrealized loss position 12 months or longer, fair value	67	89
Continuous unrealized loss position 12 months or longer, gross unrealized losses	10	7
Continuous unrealized loss position, total, fair value	5,415	1,186
Continuous unrealized loss position, total, gross unrealized losses	234	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	2
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 12 months	4
Available-for-sale securities for which fair value is less than 80% of amortized cost, total	6
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	26	26
Gross unrealized gains	6	7
Fixed maturities, at fair value	32	33
Equity securities [Member]
Investment disclosure details
Equity securities, cost	463	462
Gross unrealized gains	240	187
Gross unrealized losses	2	4
Equity securities, at fair value	701	645
Continuous unrealized loss position less than 12 months, fair value	84	53
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	4
Continuous unrealized loss position, total, fair value	84	53
Continuous unrealized loss position, total, gross unrealized losses	2	4
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	373	366
Gross unrealized gains	206	148
Gross unrealized losses	2	4
Equity securities, at fair value	577	510
Continuous unrealized loss position less than 12 months, fair value	48	40
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	4
Continuous unrealized loss position, total, fair value	48	40
Continuous unrealized loss position, total, gross unrealized losses	2	4
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	90	96
Gross unrealized gains	34	39
Equity securities, at fair value	124	135
Continuous unrealized loss position less than 12 months, fair value	36	13
Continuous unrealized loss position, total, fair value	$ 36	$ 13