Offsetting Assets and Liabilities Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 668	$ 2,298
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(71)	(1,291)
Derivative, Collateral, Right to Reclaim Cash	(469)	(841)
Derivative Liability, Fair Value, Gross Liability	(406)	(2,468)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	71	1,291
Derivative, Collateral, Obligation to Return Cash	304	1,092
Net Asset [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	545	1,395
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(34)	(503)
Derivative, Collateral, Right to Reclaim Cash	(469)	(841)
Derivative assets in net liability positions [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	37	788
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(37)	(788)
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative assets with no offsetting arrangements [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	86	115
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Derivative, Collateral, Right to Reclaim Cash	0	0
Net liability [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(362)	(1,929)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	37	788
Derivative, Collateral, Obligation to Return Cash	304	1,092
Derivative liabilities in net asset positions [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(34)	(503)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	34	503
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative liabilities with no offsetting arrangements [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(10)	(36)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
Assets [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	668	2,298
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	128	166
Assets [Member] | Net Asset [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	545	1,395
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	42	51
Assets [Member] | Derivative assets in net liability positions [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	37	788
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	0	0
Assets [Member] | Derivative assets with no offsetting arrangements [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	86	115
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	86	115
Liabilities, Total [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(406)	(2,468)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	(31)	(85)
Liabilities, Total [Member] | Net liability [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(362)	(1,929)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	(21)	(49)
Liabilities, Total [Member] | Derivative liabilities in net asset positions [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(34)	(503)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	0	0
Liabilities, Total [Member] | Derivative liabilities with no offsetting arrangements [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(10)	(36)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	$ (10)	$ (36)