Gross Amounts Associated with Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Interest Rate Swap
Derivative [Line Items]
Assets/(Liabilities)	$ 12,703	$ 15,841
Cash Collateral Posted	0	0
Net Asset/(Liability)	12,703	15,841
Futures Contracts
Derivative [Line Items]
Assets/(Liabilities)	5,617	11,148
Cash Collateral Posted	0	0
Net Asset/(Liability)	5,617	11,148
Forward Purchase Commitments
Derivative [Line Items]
Assets/(Liabilities)	2,042
Cash Collateral Posted	0
Net Asset/(Liability)	2,042
Second Interest Rate Swap
Derivative [Line Items]
Cash Collateral Posted	111,898	133,661
Net Asset/(Liability)	5,766	8,528
Assets/(Liabilities)	(106,132)	(125,133)
Second Futures Contracts
Derivative [Line Items]
Cash Collateral Posted	106,298	77,713
Net Asset/(Liability)	57,714	40,980
Assets/(Liabilities)	(48,584)	(36,733)
Forward purchase commitments including TBA dollar rolls
Derivative [Line Items]
Cash Collateral Posted	13,395	14,005
Net Asset/(Liability)	(239)	8,264
Assets/(Liabilities)	$ (13,634)	$ (5,741)