Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 224	$ 233
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(60)	(78)
Derivative, Collateral, Right to Reclaim Cash	(123)	(67)
Derivative Liability, Fair Value, Gross Liability	(124)	(145)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	60	78
Derivative, Collateral, Obligation to Return Cash	25	2
Securities Sold under Agreements to Repurchase, Gross	(725)	(648)
Non-cash derivative collateral pledged to us is not recorded on our Condensed Consolidated Balance Sheet unless certain conditions are met.	3	7
Net Asset [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	219	224
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(55)	(69)
Derivative, Collateral, Right to Reclaim Cash	(123)	(67)
Derivative assets in net liability positions [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	5	9
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(5)	(9)
Derivative, Collateral, Right to Reclaim Cash	0	0
Net liability [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(63)	(68)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	5	9
Derivative, Collateral, Obligation to Return Cash	25	2
Derivative liabilities in net asset positions [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(55)	(69)
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	55	69
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative liabilities with no offsetting arrangements [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(6)	(8)
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
Securities sold under agreement to repurchase [Member]
Derivative [Line Items]
Securities Sold under Agreements to Repurchase, Gross	(725)	(648)
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	0	0
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral, Net	(725)	(648)
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	0	0
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash	725	648
Securities Sold under Agreements to Repurchase	0	0
Assets, Total [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	224	233
Derivative Assets	41	88
Assets, Total [Member] | Net Asset [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	219	224
Derivative Assets	41	88
Assets, Total [Member] | Derivative assets in net liability positions [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	5	9
Derivative Assets	0	0
Liabilities, Total [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(124)	(145)
Derivative Liability	(39)	(65)
Gross Amounts of Recognized Liabilities	(849)	(793)
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in the Condensed Consolidated Balance Sheet	(849)	(793)
Gross Amounts Not Offset Against Collateral	60	78
Obligation to Return Cash	750	650
Net Amount	(39)	(65)
Liabilities, Total [Member] | Net liability [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(63)	(68)
Derivative Liability	(33)	(57)
Liabilities, Total [Member] | Derivative liabilities in net asset positions [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(55)	(69)
Derivative Liability	0	0
Liabilities, Total [Member] | Derivative liabilities with no offsetting arrangements [Member]
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	(6)	(8)
Derivative Liability	$ (6)	$ (8)