Derivative Financial Instruments (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012 counterparties	Dec. 31, 2011 counterparties
Derivative Financial Instruments
Cash and securities pledged as collateral by counterparties	$ 63
Securities pledged as collateral to counterparties	63
Collateral posted under MNAs, credit-risk-contingent provisions in a liability position	32	76
Collateral posted under MNAs for contracts without credit-risk-contingent liabilities	31
Credit Derivatives
Number of counter-parties	10	12
Notional amount	9,672	12,230
Credit exposure	48	108
Exposure, net of collateral	5	48
Gross liability fair value of contracts containing credit-risk-contingent features	90	153
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(48)	(69)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	(32)	(76)
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	10	8
AA-
Credit Derivatives
Number of counter-parties		1
Notional amount		25
Credit exposure		1
Exposure, net of collateral		1
A+
Credit Derivatives
Number of counter-parties	2	4
Notional amount	1,728	3,026
Credit exposure	1	26
Exposure, net of collateral	1	5
A
Credit Derivatives
Number of counter-parties	5	3
Notional amount	3,906	5,307
Credit exposure	22	15
Exposure, net of collateral	2	1
A-
Credit Derivatives
Number of counter-parties	2	2
Notional amount	421	3,815
Credit exposure	7	25
Exposure, net of collateral	2
BBB+
Credit Derivatives
Number of counter-parties	1	2
Notional amount	3,617	57
Credit exposure	18	41
Exposure, net of collateral		$ 41