Offsetting Assets and Liabilities Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 310	$ 2,298
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(102)	(1,291)
Derivative, Collateral, Right to Reclaim Cash	(158)	(841)
Derivative Liability, Fair Value, Gross Liability	(220)	(2,468)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	102	1,291
Derivative, Collateral, Obligation to Return Cash	80	1,092
Net Asset [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	233	1,395
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(56)	(503)
Derivative, Collateral, Right to Reclaim Cash	(158)	(841)
Derivative assets in net liability positions [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	46	788
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(46)	(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	31	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	(142)	(1,929)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	46	788
Derivative, Collateral, Obligation to Return Cash	80	1,092
Derivative liabilities in net asset positions [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(56)	(503)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	56	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	(22)	(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	310	2,298
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	50	166
Assets [Member] | Net Asset [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	233	1,395
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	19	51
Assets [Member] | Derivative assets in net liability positions [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	46	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	31	115
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	31	115
Liabilities, Total [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(220)	(2,468)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	(38)	(85)
Liabilities, Total [Member] | Net liability [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(142)	(1,929)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	(16)	(49)
Liabilities, Total [Member] | Derivative liabilities in net asset positions [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(56)	(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	(22)	(36)
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	$ (22)	$ (36)