Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ (26,141,370)	$ (21,998,381)	$ (26,390,596)	$ (27,401,335)	$ (33,709,043)	$ (48,115,523)
Underlying Swap [Member] | Variable Income Interest Rate [Member] | Long [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	6.0	5.9
Derivative, Notional Amount	$ (3,500,000)	$ (6,500,000)
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	3.67%	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	6.5	6.3
Derivative, Notional Amount	$ (1,700,000)	$ (2,000,000)
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	4.09%	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	5.6	5.8
Derivative, Notional Amount	$ (1,800,000)	$ (4,500,000)
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	3.27%	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	$ (3,200,000)	$ (800,000)
Derivative, Description of Variable Reference Rate	3M Libor	3M Libor
Derivative, Average Fixed Interest Rate	2.14%	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	$ (2,400,000)
Derivative, Description of Variable Reference Rate	3M Libor
Derivative, Average Fixed Interest Rate	1.71%
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	$ (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] | Fixed Income Interest Rate [Member] | Long [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	10.0
Derivative, Notional Amount	$ (1,500,000)
Derivative, Description of Variable Reference Rate	3M Libor
Derivative, Average Fixed Interest Rate	1.53%
Underlying Swap [Member] | Fixed Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Average Remaining Period Until Maturity	10.0
Derivative, Notional Amount	$ (1,500,000)
Derivative, Description of Variable Reference Rate	3M Libor
Derivative, Average Fixed Interest Rate	1.53%
Underlying Swap [Member] | Fixed Income Interest Rate [Member] | Short [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		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		10.0
Derivative, Notional Amount		$ (500,000)
Derivative, Description of Variable Reference Rate		3M Libor
Derivative, Average Fixed Interest Rate		1.75%
Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ (1,800,000)	$ (5,200,000)	$ (5,200,000)	$ (9,410,000)	$ (12,960,000)	$ (12,410,000)
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Long [Member]
Derivative [Line Items]
Derivative, Cost	(99,773)	(126,648)
Derivative, Fair Value, Net	$ (366)	$ (19,324)
Derivative, Average Remaining Period Until Maturity	11.88	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	$ (56,758)	$ (375)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ (174)
Derivative, Fair Value, Net	$ 0
Derivative, Average Remaining Period Until Maturity	1.97	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	$ (43,015)	$ (126,273)
Derivative, Fair Value, Net	$ (366)	$ (19,150)
Derivative, Average Remaining Period Until Maturity	11.88	39.17
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Short [Member]
Derivative [Line Items]
Derivative, Cost	$ (107,210)	$ (81,248)
Derivative, Fair Value, Net	$ (10,668)	$ (6,738)
Derivative, Average Remaining Period Until Maturity	2.21	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	$ (25,962)
Derivative, Fair Value, Net	$ (10,237)
Derivative, Average Remaining Period Until Maturity	2.07
Interest Rate Swaption [Member] | Variable Income Interest Rate [Member] | Short [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ (81,248)	$ (81,248)
Derivative, Fair Value, Net	$ (431)	$ (6,738)
Derivative, Average Remaining Period Until Maturity	12.01	18.01
Interest Rate Swaption [Member] | Fixed Income Interest Rate [Member] | Long [Member]
Derivative [Line Items]
Derivative, Cost	$ (20,468)
Derivative, Fair Value, Net	$ (27,362)
Derivative, Average Remaining Period Until Maturity	1.93
Interest Rate Swaption [Member] | Fixed Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	$ (20,468)
Derivative, Fair Value, Net	$ (27,362)
Derivative, Average Remaining Period Until Maturity	1.93
Interest Rate Swaption [Member] | Fixed Income Interest Rate [Member] | Short [Member]
Derivative [Line Items]
Derivative, Cost		$ (100)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ (43)
Derivative, Average Remaining Period Until Maturity		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		$ (100)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		$ (43)
Derivative, Average Remaining Period Until Maturity		0.73