Gross Amounts Associated with Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets, at fair value	$ 20,207	$ 2,914
Derivative liabilities, at fair value	(419,282)	(325,233)
Interest Rate Swap
Derivative [Line Items]
Derivative assets, at fair value		1,304
Net Asset		1,304
Interest Rate Swaptions
Derivative [Line Items]
Derivative assets, at fair value	6	727
Cash Collateral Posted (Held)	(25)	(1,106)
Net Asset	(19)	(379)
Futures Contracts
Derivative [Line Items]
Derivative assets, at fair value		693
Net Asset		693
Forward Purchase Commitments
Derivative [Line Items]
Derivative assets, at fair value	20,201	190
Net Asset	20,201	190
Second Interest Rate Swap
Derivative [Line Items]
Derivative liabilities, at fair value	(11,440)	(6,802)
Cash Collateral Posted (Held)	16,518	14,626
Net Liability	5,078	7,824
Second Futures Contracts
Derivative [Line Items]
Derivative liabilities, at fair value	(407,842)	(286,058)
Cash Collateral Posted (Held)	447,624	335,084
Net Liability	39,782	49,026
Second Forward Purchase Commitments
Derivative [Line Items]
Derivative liabilities, at fair value		(32,373)
Cash Collateral Posted (Held)	5,148	25,687
Net Liability	$ 5,148	$ (6,686)