Gross Amounts Associated with Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets, at fair value	$ 2,914	$ 27,151
Derivative liabilities, at fair value	(325,233)	(244,591)
Interest Rate Swap
Derivative [Line Items]
Derivative assets, at fair value	1,304	6,489
Net Asset	1,304	6,489
Interest Rate Swaptions
Derivative [Line Items]
Derivative assets, at fair value	727	4,561
Cash Collateral Posted (Held)	(1,106)	(1,558)
Net Asset	(379)	3,003
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	190	16,101
Net Asset	190	16,101
Second Interest Rate Swap
Derivative [Line Items]
Derivative liabilities, at fair value	(6,802)	(42,052)
Cash Collateral Posted (Held)	14,626	66,653
Net Liability	7,824	24,601
Second Futures Contracts
Derivative [Line Items]
Derivative liabilities, at fair value	(286,058)	(202,501)
Cash Collateral Posted (Held)	335,084	251,553
Net Liability	49,026	49,052
Second Forward Purchase Commitments
Derivative [Line Items]
Derivative liabilities, at fair value	(32,373)	(38)
Cash Collateral Posted (Held)	25,687	5,585
Net Liability	$ (6,686)	$ 5,547