Gross Amounts Associated with Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets	$ 28,581	$ 26,989
Interest Rate Swap
Derivative [Line Items]
Assets/(Liabilities)	4,321	15,841
Derivative assets	4,321	15,841
Interest Rate Swaptions
Derivative [Line Items]
Assets/(Liabilities)	7,321
Derivative assets	7,321
Futures Contracts
Derivative [Line Items]
Assets/(Liabilities)		11,148
Derivative assets		11,148
Forward Purchase Commitments
Derivative [Line Items]
Assets/(Liabilities)	16,939
Cash Collateral Posted	3,387	14,005
Derivative assets	20,326	8,264
Assets/(Liabilities)		(5,741)
Second Interest Rate Swap
Derivative [Line Items]
Cash Collateral Posted	111,088	133,661
Derivative assets	21,863	8,528
Assets/(Liabilities)	(89,225)	(125,133)
Second Futures Contracts
Derivative [Line Items]
Cash Collateral Posted	190,607	77,713
Derivative assets	57,635	40,980
Assets/(Liabilities)	$ (132,972)	$ (36,733)