Derivative Financial Instruments (Details 6) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 entity	Mar. 31, 2013 Credit default swaps	Dec. 31, 2012 Credit default swaps	Mar. 31, 2013 Credit default swaps Single name	Dec. 31, 2012 Credit default swaps Single name	Mar. 31, 2013 Credit default swaps Single name Investment grade corporate debt	Dec. 31, 2012 Credit default swaps Single name Investment grade corporate debt	Mar. 31, 2013 Credit default swaps Single name Municipal	Dec. 31, 2012 Credit default swaps Single name Municipal	Mar. 31, 2013 Credit default swaps Baskets	Dec. 31, 2012 Credit default swaps Baskets	Mar. 31, 2013 Credit default swaps First-to-default Municipal	Dec. 31, 2012 Credit default swaps First-to-default Municipal	Mar. 31, 2013 Credit default swaps Index Investment grade corporate debt	Dec. 31, 2012 Credit default swaps Index Investment grade corporate debt	Dec. 31, 2012 AAA Credit default swaps	Dec. 31, 2012 AAA Credit default swaps Single name	Dec. 31, 2012 AAA Credit default swaps Single name Investment grade corporate debt	Mar. 31, 2013 AA Credit default swaps	Dec. 31, 2012 AA Credit default swaps	Mar. 31, 2013 AA Credit default swaps Single name	Dec. 31, 2012 AA Credit default swaps Single name	Mar. 31, 2013 AA Credit default swaps Single name Investment grade corporate debt	Dec. 31, 2012 AA Credit default swaps Single name Investment grade corporate debt	Mar. 31, 2013 AA Credit default swaps Single name Municipal	Dec. 31, 2012 AA Credit default swaps Single name Municipal	Mar. 31, 2013 AA Credit default swaps Index Investment grade corporate debt	Dec. 31, 2012 AA Credit default swaps Index Investment grade corporate debt	Mar. 31, 2013 A Credit default swaps	Dec. 31, 2012 A Credit default swaps	Mar. 31, 2013 A Credit default swaps Single name	Dec. 31, 2012 A Credit default swaps Single name	Mar. 31, 2013 A Credit default swaps Single name Investment grade corporate debt	Dec. 31, 2012 A Credit default swaps Single name Investment grade corporate debt	Mar. 31, 2013 A Credit default swaps Baskets	Dec. 31, 2012 A Credit default swaps Baskets	Mar. 31, 2013 A Credit default swaps First-to-default Municipal	Dec. 31, 2012 A Credit default swaps First-to-default Municipal	Mar. 31, 2013 A Credit default swaps Index Investment grade corporate debt	Dec. 31, 2012 A Credit default swaps Index Investment grade corporate debt	Mar. 31, 2013 BBB Credit default swaps	Dec. 31, 2012 BBB Credit default swaps	Mar. 31, 2013 BBB Credit default swaps Single name	Dec. 31, 2012 BBB Credit default swaps Single name	Mar. 31, 2013 BBB Credit default swaps Single name Investment grade corporate debt	Dec. 31, 2012 BBB Credit default swaps Single name Investment grade corporate debt	Mar. 31, 2013 BBB Credit default swaps Index Investment grade corporate debt	Dec. 31, 2012 BBB Credit default swaps Index Investment grade corporate debt	Mar. 31, 2013 BB and lower Credit default swaps	Dec. 31, 2012 BB and lower Credit default swaps	Dec. 31, 2012 BB and lower Credit default swaps Single name	Dec. 31, 2012 BB and lower Credit default swaps Single name Investment grade corporate debt	Mar. 31, 2013 BB and lower Credit default swaps Index Investment grade corporate debt	Dec. 31, 2012 BB and lower Credit default swaps Index Investment grade corporate debt
Derivative Financial Instruments
Term of credit default swaps (in years)	P5Y
The number of reference entities generally included in a CDX index	125
Credit Derivatives
Notional amount		$ 335	$ 593	$ 135	$ 193	$ 110	$ 168	$ 25	$ 25	$ 100	$ 100	$ 100	$ 100	$ 100	$ 300	$ 5	$ 5	$ 5	$ 46	$ 48	$ 45	$ 45	$ 20	$ 20	$ 25	$ 25	$ 1	$ 3	$ 156	$ 232	$ 30	$ 53	$ 30	$ 53	$ 100	$ 100	$ 100	$ 100	$ 26	$ 79	$ 127	$ 284	$ 60	$ 80	$ 60	$ 80	$ 67	$ 204	$ 6	$ 24	$ 10	$ 10	$ 6	$ 14
Fair value		$ (21)	$ (27)	$ (1)	$ (3)	$ 1		$ (2)	$ (3)	$ (21)	$ (26)	$ (21)	$ (26)	$ 1	$ 2