Derivative Financial Instruments (Details 5) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 party	Dec. 31, 2012 party
Derivative Financial Instruments
Cash and securities pledged as collateral by counterparties	$ 33
Securities pledged as collateral to counterparties	26
Credit Derivatives
Number of counter-parties	8	7
Notional amount	6,603	6,199
Credit exposure	31	24
Exposure, net of collateral	1	1
Gross liability fair value of contracts containing credit-risk-contingent features	53	62
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(25)	(28)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	(26)	(25)
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	2	9
A+
Credit Derivatives
Number of counter-parties	1	1
Notional amount	883	19
Credit exposure	3
Exposure, net of collateral	1
A
Credit Derivatives
Number of counter-parties	4	3
Notional amount	2,270	2,252
Credit exposure	14	12
A-
Credit Derivatives
Number of counter-parties	2	2
Notional amount	90	311
Credit exposure		1
Exposure, net of collateral		1
BBB+
Credit Derivatives
Number of counter-parties	1	1
Notional amount	3,360	3,617
Credit exposure	$ 14	$ 11