Investments - Appreciation (Depreciation) on Fixed Maturities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Gross Unrealized Appreciation (Depreciation) on Fixed Maturities By Type [Abstract]
Federal government and agency	$ 847	$ 902
State and local government	2,401	2,437
Foreign government	1,242	1,322
Corporate	10,991	11,881
Federal agency mortgage-backed	107	122
Other mortgage-backed	83	89
Other asset-backed	913	936
Total	16,584	17,689
Portion At Amortized Cost Disclosure [Member]
Gross Unrealized Appreciation (Depreciation) on Fixed Maturities By Type [Abstract]
Federal government and agency	482	509
State and local government	2,148	2,169
Foreign government	1,125	1,197
Corporate	9,797	10,590
Federal agency mortgage-backed	106	121
Other mortgage-backed	77	82
Other asset-backed	780	797
Total	14,515	15,465
Unrealized Gain [Member]
Gross Unrealized Appreciation (Depreciation) on Fixed Maturities By Type [Abstract]
Federal government and agency	365	393
State and local government	254	270
Foreign government	118	126
Corporate	1,212	1,308
Federal agency mortgage-backed	1	1
Other mortgage-backed	10	11
Other asset-backed	135	145
Total	2,095	2,254
Unrealized Loss Or Write Down [Member]
Gross Unrealized Appreciation (Depreciation) on Fixed Maturities By Type [Abstract]
Federal government and agency	0	0
State and local government	(1)	(2)
Foreign government	(1)	(1)
Corporate	(18)	(17)
Federal agency mortgage-backed	0	0
Other mortgage-backed	(4)	(4)
Other asset-backed	(2)	(6)
Total	$ (26)	$ (30)