Gross Amounts Associated with Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Interest Rate Swap
Derivative [Line Items]
Assets/(Liabilities)	$ 6,489	$ 15,841
Net Asset/(Liability)	6,489	15,841
Interest Rate Swaptions
Derivative [Line Items]
Assets/(Liabilities)	4,561
Cash Collateral Posted (Held)	(1,558)
Net Asset/(Liability)	3,003
Second Forward Purchase Commitments
Derivative [Line Items]
Assets/(Liabilities)	16,101
Net Asset/(Liability)	16,101
Second Interest Rate Swap
Derivative [Line Items]
Cash Collateral Posted (Held)	66,653	133,661
Net Asset/(Liability)	24,601	8,528
Assets/(Liabilities)	(42,052)	(125,133)
Forward Purchase Commitments
Derivative [Line Items]
Cash Collateral Posted (Held)	5,585	14,005
Net Asset/(Liability)	5,547	8,264
Assets/(Liabilities)	(38)	(5,741)
Second Futures Contracts
Derivative [Line Items]
Cash Collateral Posted (Held)	251,553	77,713
Net Asset/(Liability)	49,052	40,980
Assets/(Liabilities)	(202,501)	(36,733)
Futures Contracts
Derivative [Line Items]
Assets/(Liabilities)		11,148
Net Asset/(Liability)		$ 11,148

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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