Gross Amounts Associated with Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets, at fair value	$ 4,365	$ 27,151
Derivative liabilities, at fair value	418,921	244,591
Interest Rate Swaptions
Derivative [Line Items]
Derivative assets, at fair value	1,831	4,561
Cash Collateral Posted (Held)	(886)	(1,558)
Net Asset	945	3,003
Forward Purchase Commitments
Derivative [Line Items]
Derivative assets, at fair value	2,534	16,101
Net Asset	2,534	16,101
Second Interest Rate Swap
Derivative [Line Items]
Derivative liabilities, at fair value	(22,964)	(42,052)
Cash Collateral Posted (Held)	31,626	66,653
Net Liability	8,662	24,601
Second Futures Contracts
Derivative [Line Items]
Derivative liabilities, at fair value	(394,221)	(202,501)
Cash Collateral Posted (Held)	458,178	251,553
Net Liability	63,957	49,052
Second Forward Purchase Commitments
Derivative [Line Items]
Derivative liabilities, at fair value	(1,736)	(38)
Cash Collateral Posted (Held)	3,217	5,585
Net Liability	$ 1,481	5,547
Interest Rate Swap
Derivative [Line Items]
Derivative assets, at fair value		6,489
Net Asset		$ 6,489