Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Long [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	$ 232,594
Derivative Asset, Fair Value, Net	221,247
Derivative, Weighted Average Remaining Maturity	36.188
Derivative, Weighted Average Remaining Maturity	36.188
Long [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	9.118
Derivative, Notional Amount	6,800,000
Derivative, Average Fixed Interest Rate	4.19%
Derivative, Description of Variable Rate Basis	3M Libor
Derivative, Weighted Average Remaining Maturity	9.118
Long [Member] | Six Months or Longer Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	223,504
Derivative Asset, Fair Value, Net	219,922
Derivative, Weighted Average Remaining Maturity	36.490
Derivative, Weighted Average Remaining Maturity	36.490
Long [Member] | Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	9
Derivative, Notional Amount	6,000,000
Derivative, Average Fixed Interest Rate	4.274%
Derivative, Description of Variable Rate Basis	3M Libor
Derivative, Weighted Average Remaining Maturity	9
Short [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	6,938
Derivative Asset, Fair Value, Net	4,461
Derivative, Weighted Average Remaining Maturity	4.505
Derivative, Weighted Average Remaining Maturity	4.505
Short [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	5
Derivative, Notional Amount	4,000,000
Derivative, Average Fixed Interest Rate	1.378%
Derivative, Description of Variable Rate Basis	3M Libor
Derivative, Weighted Average Remaining Maturity	5
Short [Member] | Less Than Six Months Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	6,038
Derivative Asset, Fair Value, Net	3,939
Derivative, Weighted Average Remaining Maturity	3.233
Derivative, Weighted Average Remaining Maturity	3.233
Short [Member] | Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	5
Derivative, Notional Amount	2,000,000
Derivative, Average Fixed Interest Rate	1.675%
Derivative, Description of Variable Rate Basis	3M Libor
Derivative, Weighted Average Remaining Maturity	5
Short [Member] | Six Months or Longer Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	900
Derivative Asset, Fair Value, Net	522
Derivative, Weighted Average Remaining Maturity	9.3
Derivative, Weighted Average Remaining Maturity	9.3
Short [Member] | Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	5
Derivative, Notional Amount	2,000,000
Derivative, Average Fixed Interest Rate	1.08%
Derivative, Description of Variable Rate Basis	3M Libor
Derivative, Weighted Average Remaining Maturity	5
Variable Income Interest Rate [Member] | Long [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost		233,935
Derivative Asset, Fair Value, Net		363,566
Derivative, Weighted Average Remaining Maturity		38.162
Derivative, Weighted Average Remaining Maturity		38.162
Variable Income Interest Rate [Member] | Long [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity		9.101
Derivative, Notional Amount		6,675,000
Derivative, Average Fixed Interest Rate		4.178%
Derivative, Description of Variable Rate Basis		3M Libor
Derivative, Weighted Average Remaining Maturity		9.101
Variable Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost		10,431
Derivative Asset, Fair Value, Net		10,458
Derivative, Weighted Average Remaining Maturity		2.776
Derivative, Weighted Average Remaining Maturity		2.776
Variable Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity		10
Derivative, Notional Amount		675,000
Derivative, Average Fixed Interest Rate		3.326%
Derivative, Description of Variable Rate Basis		3M Libor
Derivative, Weighted Average Remaining Maturity		10
Variable Income Interest Rate [Member] | Long [Member] | Six Months or Longer Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost		223,504
Derivative Asset, Fair Value, Net		353,108
Derivative, Weighted Average Remaining Maturity		39.144
Derivative, Weighted Average Remaining Maturity		39.144
Variable Income Interest Rate [Member] | Long [Member] | Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity		9
Derivative, Notional Amount		6,000,000
Derivative, Average Fixed Interest Rate		4.274%
Derivative, Description of Variable Rate Basis		3M Libor
Derivative, Weighted Average Remaining Maturity		9
Variable Income Interest Rate [Member] | Short [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	(81,248)	(84,703)
Derivative Asset, Fair Value, Net	(58,645)	(94,040)
Derivative, Weighted Average Remaining Maturity	39.022	33.679
Derivative, Weighted Average Remaining Maturity	39.022	33.679
Variable Income Interest Rate [Member] | Short [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	10	8.053
Derivative, Notional Amount	(800,000)	(1,310,000)
Derivative, Average Fixed Interest Rate	3.438%	2.722%
Derivative, Description of Variable Rate Basis	3M Libor	3M Libor
Derivative, Weighted Average Remaining Maturity	10	8.053
Variable Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost		(3,455)
Derivative Asset, Fair Value, Net		(7,679)
Derivative, Weighted Average Remaining Maturity		1.933
Derivative, Weighted Average Remaining Maturity		1.933
Variable Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity		5
Derivative, Notional Amount		(510,000)
Derivative, Average Fixed Interest Rate		1.60%
Derivative, Description of Variable Rate Basis		3M Libor
Derivative, Weighted Average Remaining Maturity		5
Variable Income Interest Rate [Member] | Short [Member] | Six Months or Longer Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	(81,248)	(81,248)
Derivative Asset, Fair Value, Net	(58,645)	(86,361)
Derivative, Weighted Average Remaining Maturity	39.022	42.023
Derivative, Weighted Average Remaining Maturity	39.022	42.023
Variable Income Interest Rate [Member] | Short [Member] | Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	10	10
Derivative, Notional Amount	(800,000)	(800,000)
Derivative, Average Fixed Interest Rate	3.438%	3.438%
Derivative, Description of Variable Rate Basis	3M Libor	3M Libor
Derivative, Weighted Average Remaining Maturity	10	10
Fixed Income Interest Rate [Member] | Long [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost		3,991
Derivative Asset, Fair Value, Net		681
Derivative, Weighted Average Remaining Maturity		1.933
Derivative, Weighted Average Remaining Maturity		1.933
Fixed Income Interest Rate [Member] | Long [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity		10
Derivative, Notional Amount		275,000
Derivative, Average Fixed Interest Rate		2.89%
Derivative, Description of Variable Rate Basis		3M Libor
Derivative, Weighted Average Remaining Maturity		10
Fixed Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost		3,991
Derivative Asset, Fair Value, Net		681
Derivative, Weighted Average Remaining Maturity		1.933
Derivative, Weighted Average Remaining Maturity		1.933
Fixed Income Interest Rate [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity		10
Derivative, Notional Amount		275,000
Derivative, Average Fixed Interest Rate		2.89%
Derivative, Description of Variable Rate Basis		3M Libor
Derivative, Weighted Average Remaining Maturity		10
Fixed Income Interest Rate [Member] | Short [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	(2,625)	(3,455)
Derivative Asset, Fair Value, Net	(2,767)	(462)
Derivative, Weighted Average Remaining Maturity	2.3	1.933
Derivative, Weighted Average Remaining Maturity	2.3	1.933
Fixed Income Interest Rate [Member] | Short [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	10	5
Derivative, Notional Amount	(500,000)	(510,000)
Derivative, Average Fixed Interest Rate	3.20%	1.60%
Derivative, Description of Variable Rate Basis	3M Libor	3M Libor
Derivative, Weighted Average Remaining Maturity	10	5
Fixed Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member] | Interest Rate Swaption [Member]
Derivative [Line Items]
Derivative, Cost	(2,625)	(3,455)
Derivative Asset, Fair Value, Net	(2,767)	(462)
Derivative, Weighted Average Remaining Maturity	2.3	1.933
Derivative, Weighted Average Remaining Maturity	2.3	1.933
Fixed Income Interest Rate [Member] | Short [Member] | Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	10	5
Derivative, Notional Amount	(500,000)	(510,000)
Derivative, Average Fixed Interest Rate	3.20%	1.60%
Derivative, Description of Variable Rate Basis	3M Libor	3M Libor
Derivative, Weighted Average Remaining Maturity	10	5
Options Held [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Cost	9,090
Derivative Asset, Fair Value, Net	1,325
Derivative, Weighted Average Remaining Maturity	2.293
Derivative, Weighted Average Remaining Maturity	2.293
Underlying Swap [Member] | Long [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Derivative, Weighted Average Remaining Maturity	10
Derivative, Notional Amount	$ 800,000
Derivative, Average Fixed Interest Rate	3.563%
Derivative, Description of Variable Rate Basis	3M Libor
Derivative, Weighted Average Remaining Maturity	10