Derivative Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Components of unrealized net gains and net interest income from Linked Transactions
Change in fair value of Linked Transactions included in earnings			$ (8,318)	$ 8,851
Linked Transactions
Linked Repurchase Agreements
Balance	82,417		82,417		35,271
Weighted Average Interest Rate (as a percent)	1.69%		1.69%		1.63%
Linked MBS
Fair Value	106,231		106,231		47,828
Amortized Cost	102,312		102,312		43,817
Par/Current Face	110,664		110,664		52,277
Weighted Average Coupon Rate (as a percent)			3.99%		4.28%
Associated accrued interest receivable	186		186		185
Associated accrued interest payable	60		60		38
Components of unrealized net gains and net interest income from Linked Transactions
Interest income attributable to MBS underlying Linked Transactions	1,109	812	2,431	4,386
Interest expense attributable to linked repurchase agreement borrowings underlying Linked Transactions	(275)	(168)	(552)	(965)
Change in fair value of Linked Transactions included in earnings	(290)	2,533	(94)	8,023
Unrealized net gains and net interest income from Linked Transactions	544	3,177	1,785	11,444
Linked Transactions | Rated AA
Linked MBS
Fair Value	9,724		9,724		13,588
Amortized Cost	9,162		9,162		12,817
Par/Current Face	9,487		9,487		13,192
Weighted Average Coupon Rate (as a percent)			5.00%		5.00%
Linked Transactions | Rated A
Linked MBS
Fair Value	2,654		2,654		3,075
Amortized Cost	2,138		2,138		2,548
Par/Current Face	2,793		2,793		3,342
Weighted Average Coupon Rate (as a percent)			1.28%		0.76%
Linked Transactions | Rated BBB
Linked MBS
Fair Value	7,672		7,672		8,299
Amortized Cost	6,730		6,730		7,226
Par/Current Face	8,149		8,149		8,847
Weighted Average Coupon Rate (as a percent)			2.31%		2.55%
Linked Transactions | Rated BB
Linked MBS
Fair Value	6,637		6,637		7,365
Amortized Cost	6,177		6,177		6,854
Par/Current Face	6,773		6,773		7,593
Weighted Average Coupon Rate (as a percent)			2.56%		2.75%
Linked Transactions | Rated D
Linked MBS
Fair Value	13,921		13,921		15,501
Amortized Cost	12,450		12,450		14,372
Par/Current Face	16,939		16,939		19,303
Weighted Average Coupon Rate (as a percent)			5.80%		5.80%
Linked Transactions | Rated NR
Linked MBS
Fair Value	65,623		65,623
Amortized Cost	65,655		65,655
Par/Current Face	66,523		66,523
Weighted Average Coupon Rate (as a percent)			3.85%
Linked Transactions | Within 30 days
Linked Repurchase Agreements
Balance	61,642		61,642		13,672
Weighted Average Interest Rate (as a percent)	1.72%		1.72%		1.57%
Linked Transactions | Over 30 days to 90 days
Linked Repurchase Agreements
Balance	$ 20,775		$ 20,775		$ 21,599
Weighted Average Interest Rate (as a percent)	1.61%		1.61%		1.66%