Gross Amounts Associated with Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets	$ 31,002	$ 26,989
Derivative liabilities	175,322	167,607
Interest Rate Swap
Derivative [Line Items]
Assets/(Liabilities)	11,716	15,841
Derivative assets	11,716	15,841
Interest Rate Swaptions
Derivative [Line Items]
Assets/(Liabilities)	16,099
Cash Collateral Posted (Held)	(3,598)
Derivative assets	12,501
Futures Contracts
Derivative [Line Items]
Assets/(Liabilities)	1,299	11,148
Derivative assets	1,299	11,148
Second Forward Purchase Commitments
Derivative [Line Items]
Assets/(Liabilities)	1,888
Derivative assets	1,888
Second Interest Rate Swap
Derivative [Line Items]
Cash Collateral Posted (Held)	63,511	133,661
Derivative assets	5,027	8,528
Assets/(Liabilities)	(58,484)	(125,133)
Forward Purchase Commitments
Derivative [Line Items]
Cash Collateral Posted (Held)	9,263	14,005
Derivative assets		8,264
Assets/(Liabilities)	(19,971)	(5,741)
Derivative liabilities	(10,708)
Second Futures Contracts
Derivative [Line Items]
Cash Collateral Posted (Held)	153,990	77,713
Derivative assets	57,123	40,980
Assets/(Liabilities)	$ (96,867)	$ (36,733)