Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 263	$ 362
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(107)	(108)
Derivative, Collateral, Right to Reclaim Cash	(68)	(120)
Derivative Liability, Fair Value, Gross Liability	(252)	(317)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	107	108
Derivative, Collateral, Obligation to Return Cash	54	137
Securities Sold under Agreements to Repurchase, Gross	(774)	(1,500)
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	0	0
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral, Net	(774)	(1,500)
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	0	0
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash	774	1,500
Securities Sold under Agreements to Repurchase	0	0
Net Asset [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	216	319
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(60)	(65)
Derivative, Collateral, Right to Reclaim Cash	(68)	(120)
Derivative assets in net liability positions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	47	43
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(47)	(43)
Derivative, Collateral, Right to Reclaim Cash	0	0
Net liability [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	(188)	(252)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	47	43
Derivative, Collateral, Obligation to Return Cash	54	137
Derivative liabilities in net asset positions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	(60)	(65)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	60	65
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative liabilities with no offsetting arrangements [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	(4)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0
Derivative, Collateral, Obligation to Return Cash	0
Assets [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	263	362
Derivative Assets	88	134
Assets [Member] | Net Asset [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	216	319
Derivative Assets	88	134
Assets [Member] | Derivative assets in net liability positions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	47	43
Derivative Assets	0	0
Liabilities, Total [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(252)	(317)
Derivative Liability	(91)	(72)
Gross Amounts of Recognized Liabilities	(1,026)	(1,817)
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in the Condensed Consolidated Balance Sheet	(1,026)	(1,817)
Gross Amounts Not Offset Against Collateral	107	108
Obligation to Return Cash	828	1,637
Net Amount	(91)	(72)
Liabilities, Total [Member] | Net liability [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(188)	(252)
Derivative Liability	(87)	(72)
Liabilities, Total [Member] | Derivative liabilities in net asset positions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(60)	(65)
Derivative Liability	0	0
Liabilities, Total [Member] | Derivative liabilities with no offsetting arrangements [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(4)
Derivative Liability	$ (4)