Derivatives and Hedging Activities (Details 7) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Credit risk contingent feature
Additional collateral aggregate fair value	$ 231
One Rating Downgrade [Member] | Moody's, A3 Rating [Member]
Credit risk contingent feature
Additional collateral aggregate fair value	6	11	11
One Rating Downgrade [Member] | Standard & Poor's, A- Rating [Member]
Credit risk contingent feature
Additional collateral aggregate fair value	6	11	11
Two Rating Downgrade [Member] | Moody's, A3 Rating [Member]
Credit risk contingent feature
Additional collateral aggregate fair value	11	16	16
Two Rating Downgrade [Member] | Standard & Poor's, A- Rating [Member]
Credit risk contingent feature
Additional collateral aggregate fair value	11	16	16
Three Rating Downgrade [Member] | Moody's, A3 Rating [Member]
Credit risk contingent feature
Additional collateral aggregate fair value	17	16	16
Three Rating Downgrade [Member] | Standard & Poor's, A- Rating [Member]
Credit risk contingent feature
Additional collateral aggregate fair value	$ 17	$ 16	$ 16