Investments (details) - Investment Information (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment disclosure details
Fixed maturities, amortized cost	$ 59,960	$ 60,829
Fixed maturities, at fair value	62,102	65,393
Equity securities, cost	470	462
Equity securities, at fair value	708	645
Continuous unrealized loss position less than 12 months, fair value	11,492	2,606
Continuous unrealized loss position less than 12 months, gross unrealized losses	587	27
Continuous unrealized loss position 12 months or longer, fair value	140	172
Continuous unrealized loss position 12 months or longer, gross unrealized losses	17	11
Continuous unrealized loss position, total, fair value	11,632	2,778
Continuous unrealized loss position, total, gross unrealized losses	604	38
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	6
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 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	12
Fixed maturities [Member]
Investment disclosure details
Fixed maturities, amortized cost	59,960	60,829
Gross unrealized gains	2,744	4,598
Gross unrealized losses	602	34
Fixed maturities, at fair value	62,102	65,393
Continuous unrealized loss position less than 12 months, fair value	11,427	2,553
Continuous unrealized loss position less than 12 months, gross unrealized losses	585	23
Continuous unrealized loss position 12 months or longer, fair value	140	172
Continuous unrealized loss position 12 months or longer, gross unrealized losses	17	11
Continuous unrealized loss position, total, fair value	11,567	2,725
Continuous unrealized loss position, total, gross unrealized losses	602	34
Available-for-sale securities for which fair value is less than 80% of amortized cost, 3 months or less	5
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 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	11
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,069	2,148
Gross unrealized gains	48	75
Gross unrealized losses	8	1
Fixed maturities, at fair value	2,109	2,222
Continuous unrealized loss position less than 12 months, fair value	238	589
Continuous unrealized loss position less than 12 months, gross unrealized losses	8	1
Continuous unrealized loss position, total, fair value	238	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	34,958	35,863
Gross unrealized gains	1,626	2,829
Gross unrealized losses	334	11
Fixed maturities, at fair value	36,250	38,681
Continuous unrealized loss position less than 12 months, fair value	4,852	611
Continuous unrealized loss position less than 12 months, gross unrealized losses	333	9
Continuous unrealized loss position 12 months or longer, fair value	11	45
Continuous unrealized loss position 12 months or longer, gross unrealized losses	1	2
Continuous unrealized loss position, total, fair value	4,863	656
Continuous unrealized loss position, total, gross unrealized losses	334	11
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	9,141	8,458
Gross unrealized gains	489	567
Gross unrealized losses	1
Fixed maturities, at fair value	9,629	9,025
Obligations of states, municipalities and political subdivisions, all other (not pre-refunded) [Member]
Investment disclosure details
Fixed maturities, amortized cost	25,817	27,405
Gross unrealized gains	1,137	2,262
Gross unrealized losses	333	11
Fixed maturities, at fair value	26,621	29,656
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,937	2,185
Gross unrealized gains	40	72
Gross unrealized losses	3
Fixed maturities, at fair value	1,974	2,257
Continuous unrealized loss position less than 12 months, fair value	420	186
Continuous unrealized loss position less than 12 months, gross unrealized losses	3
Continuous unrealized loss position 12 months or longer, fair value		2
Continuous unrealized loss position, total, fair value	420	188
Continuous unrealized loss position, total, gross unrealized losses	3
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,337	2,744
Gross unrealized gains	191	255
Gross unrealized losses	12	2
Fixed maturities, at fair value	2,516	2,997
Continuous unrealized loss position less than 12 months, fair value	436	70
Continuous unrealized loss position less than 12 months, gross unrealized losses	10
Continuous unrealized loss position 12 months or longer, fair value	22	36
Continuous unrealized loss position 12 months or longer, gross unrealized losses	2	2
Continuous unrealized loss position, total, fair value	458	106
Continuous unrealized loss position, total, gross unrealized losses	12	2
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	18,633	17,863
Gross unrealized gains	833	1,360
Gross unrealized losses	245	20
Fixed maturities, at fair value	19,221	19,203
Continuous unrealized loss position less than 12 months, fair value	5,481	1,097
Continuous unrealized loss position less than 12 months, gross unrealized losses	231	13
Continuous unrealized loss position 12 months or longer, fair value	107	89
Continuous unrealized loss position 12 months or longer, gross unrealized losses	14	7
Continuous unrealized loss position, total, fair value	5,588	1,186
Continuous unrealized loss position, total, gross unrealized losses	245	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	5
Available-for-sale securities for which fair value is less than 80% of amortized cost, greater than 3 months, 6 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	11
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	470	462
Gross unrealized gains	240	187
Gross unrealized losses	2	4
Equity securities, at fair value	708	645
Continuous unrealized loss position less than 12 months, fair value	65	53
Continuous unrealized loss position less than 12 months, gross unrealized losses	2	4
Continuous unrealized loss position, total, fair value	65	53
Continuous unrealized loss position, total, gross unrealized losses	2	4
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, total	1
Equity securities, common stock [Member]
Investment disclosure details
Equity securities, cost	380	366
Gross unrealized gains	205	148
Gross unrealized losses	1	4
Equity securities, at fair value	584	510
Continuous unrealized loss position less than 12 months, fair value	37	40
Continuous unrealized loss position less than 12 months, gross unrealized losses	1	4
Continuous unrealized loss position, total, fair value	37	40
Continuous unrealized loss position, total, gross unrealized losses	1	4
Equity securities, non-redeemable preferred stock [Member]
Investment disclosure details
Equity securities, cost	90	96
Gross unrealized gains	35	39
Gross unrealized losses	1
Equity securities, at fair value	124	135
Continuous unrealized loss position less than 12 months, fair value	28	13
Continuous unrealized loss position less than 12 months, gross unrealized losses	1
Continuous unrealized loss position, total, fair value	28	13
Continuous unrealized loss position, total, gross unrealized losses	$ 1