Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 18,802,500	$ 21,998,381	$ 48,115,523
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Long [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	5.1	5.9
Derivative, Notional Amount	$ 4,800,000	$ 6,500,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	2.24%	3.24%
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	4.8	6.3
Derivative, Notional Amount	$ 4,500,000	$ 2,000,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	2.16%	2.23%
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Long [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	10.0	5.8
Derivative, Notional Amount	$ 300,000	$ 4,500,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	3.50%	3.69%
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Short [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	10.0	10.0
Derivative, Notional Amount	$ 800,000	$ 800,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	3.44%	3.44%
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	10.0
Derivative, Notional Amount	$ 500,000
Derivative, Description of Variable Reference Rate	3M Libor
Derivative, Average Fixed Interest Rate	3.40%
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Short [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	10.0	10.0
Derivative, Notional Amount	$ 300,000	$ 800,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	3.50%	3.44%
Underlying Swap [Member] | Fixed Income Interest Rate [Member] | Short [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	4.9	10.0
Derivative, Notional Amount	$ 3,775,000	$ 500,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	1.19%	1.75%
Underlying Swap [Member] | Fixed Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	4.9	10.0
Derivative, Notional Amount	$ 3,775,000	$ 500,000
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	1.19%	1.75%
Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 225,000	$ 5,200,000	$ 12,410,000
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Long [Member]
Derivative [Line Items]
Derivative, Cost	(43,015)	(126,648)
Derivative, Fair Value, Net	$ (42,941)	$ (19,324)
Derivative, Average Remaining Period Until Maturity	1.23	38.51
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ (29,360)	$ (375)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ (174)
Derivative, Fair Value, Net	$ (42,149)
Derivative, Average Remaining Period Until Maturity	1.22	0.75
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Long [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ (13,655)	$ (126,273)
Derivative, Fair Value, Net	$ (792)	$ (19,150)
Derivative, Average Remaining Period Until Maturity	6.70	39.17
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Short [Member]
Derivative [Line Items]
Derivative, Cost	$ (81,248)	$ (81,248)
Derivative, Fair Value, Net	$ (2,352)	$ (6,738)
Derivative, Average Remaining Period Until Maturity	6.05	18.01
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ (51,355)
Derivative, Fair Value, Net	$ (1,414)
Derivative, Average Remaining Period Until Maturity	5.81
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Short [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ (29,893)	$ (81,248)
Derivative, Fair Value, Net	$ (938)	$ (6,738)
Derivative, Average Remaining Period Until Maturity	6.77	18.01
Interest Rate Swaption [Member] | Fixed Income Interest Rate [Member] | Short [Member]
Derivative [Line Items]
Derivative, Cost	$ 0	$ (100)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ (2,353)	$ (43)
Derivative, Average Remaining Period Until Maturity	2.30	0.73
Interest Rate Swaption [Member] | Fixed Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ 0	$ (100)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ (2,353)	$ (43)
Derivative, Average Remaining Period Until Maturity	2.30	0.73