Derivative Financial Instruments (Details 5) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 counterparties	Dec. 31, 2012 counterparties
Derivative Financial Instruments
Cash and securities pledged as collateral by counterparties	$ 33
Securities pledged as collateral to counterparties	29
Collateral posted under MNAs, credit-risk-contingent provisions in a liability position	26	25
Collateral posted under MNAs for contracts without credit-risk-contingent liabilities	3
Credit Derivatives
Number of counter-parties	9	10
Notional amount	6,817	6,893
Credit exposure	33	33
Exposure, net of collateral	2	5
Gross liability fair value of contracts containing credit-risk-contingent features	55	65
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(26)	(31)
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	3	9
A+
Credit Derivatives
Number of counter-parties	1	2
Notional amount	918	29
Credit exposure	3	1
Exposure, net of collateral	1	1
A
Credit Derivatives
Number of counter-parties	4	4
Notional amount	2,270	2,450
Credit exposure	14	13
Exposure, net of collateral		2
A-
Credit Derivatives
Number of counter-parties	3	3
Notional amount	269	797
Credit exposure	2	8
Exposure, net of collateral	1	2
BBB+
Credit Derivatives
Number of counter-parties	1	1
Notional amount	3,360	3,617
Credit exposure	$ 14	$ 11