Derivative Financial Instruments and Off-balance sheet Financial Instruments (Details 4) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 counterparties	Dec. 31, 2011 counterparties
Derivative Financial Instruments and Off-balance sheet Financial Instruments
Cash and securities pledged as collateral by counterparties	$ 29
Securities pledged as collateral to counterparties	26
Collateral posted under MNAs, credit-risk-contingent provisions in a liability position	25	76
Collateral posted under MNAs for contracts without credit-risk-contingent liabilities	1
Credit Derivatives
Number of counter-parties	10	12
Notional amount	6,893	12,230
Credit exposure	33	108
Exposure, net of collateral	5	48
Gross liability fair value of contracts containing credit-risk-contingent features	65	153
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(31)	(69)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	(25)	(76)
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	9	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	29	3,026
Credit exposure	1	26
Exposure, net of collateral	1	5
A
Credit Derivatives
Number of counter-parties	4	3
Notional amount	2,450	5,307
Credit exposure	13	15
Exposure, net of collateral	2	1
A-
Credit Derivatives
Number of counter-parties	3	2
Notional amount	797	3,815
Credit exposure	8	25
Exposure, net of collateral	2
BBB+
Credit Derivatives
Number of counter-parties	1	2
Notional amount	3,617	57
Credit exposure	11	41
Exposure, net of collateral		$ 41