Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Components of unrealized net gains and net interest income from Linked Transactions
Change in fair value of Linked Transactions included in earnings	$ 2,076	$ 1,136
Linked Transactions
Linked Repurchase Agreements
Balance	205,986		102,737
Weighted Average Interest Rate (as a percent)	1.72%		1.73%
Linked MBS
Fair Value	265,548		130,790
Amortized Cost	259,493		126,497
Par/Current Face	272,375		134,430
Weighted Average Coupon Rate (as a percent)	3.92%		3.96%
Associated accrued interest receivable	423		210
Associated accrued interest payable	159		82
Components of unrealized net gains and net interest income from Linked Transactions
Interest income attributable to MBS underlying Linked Transactions	2,041	668
Interest expense attributable to linked repurchase agreement borrowings underlying Linked Transactions	(551)	(140)
Change in Fair Value of Linked Transactions Included in Earnings	1,761	1,008
Unrealized net gains and net interest income from Linked Transactions	3,251	1,536
Linked Transactions | Within 30 days
Linked Repurchase Agreements
Balance	173,386		93,835
Weighted Average Interest Rate (as a percent)	1.75%		1.76%
Linked Transactions | Over 30 days to 90 days
Linked Repurchase Agreements
Balance	$ 32,600		$ 8,902
Weighted Average Interest Rate (as a percent)	1.57%		1.44%