Derivatives (Details 2) (Linked Transactions, USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Linked Repurchase Agreements
Balance	$ 34,136		$ 35,271
Weighted Average Interest Rate (as a percent)	1.60%		1.63%
Linked MBS
Fair Value	46,583		47,828
Amortized Cost	41,564		43,817
Par/Current Face	49,635		52,277
Weighted Average Coupon Rate (as a percent)	4.27%		4.28%
Components of the gain on Linked Transactions
Interest income attributable to MBS underlying Linked Transactions	668	2,288
Interest expense attributable to linked repurchase agreement borrowings underlying Linked Transactions	(140)	(504)
Change in fair value of Linked Transactions included in earnings	1,008	5,915
Unrealized net gains and net interest income from Linked Transactions	1,536	7,699
Rated AA
Linked MBS
Fair Value	12,654		13,588
Amortized Cost	11,578		12,817
Par/Current Face	11,937		13,192
Weighted Average Coupon Rate (as a percent)	5.00%		5.00%
Rated A
Linked MBS
Fair Value	3,019		3,075
Amortized Cost	2,431		2,548
Par/Current Face	3,178		3,342
Weighted Average Coupon Rate (as a percent)	0.75%		0.76%
Rated BBB
Linked MBS
Fair Value	8,270		8,299
Amortized Cost	7,121		7,226
Par/Current Face	8,680		8,847
Weighted Average Coupon Rate (as a percent)	2.53%		2.55%
Rated BB
Linked MBS
Fair Value	7,162		7,365
Amortized Cost	6,540		6,854
Par/Current Face	7,234		7,593
Weighted Average Coupon Rate (as a percent)	2.70%		2.75%
D-rated
Linked MBS
Fair Value	15,478		15,501
Amortized Cost	13,894		14,372
Par/Current Face	18,606		19,303
Weighted Average Coupon Rate (as a percent)	5.82%		5.80%
Within 30 days
Linked Repurchase Agreements
Balance	23,236		13,672
Weighted Average Interest Rate (as a percent)	1.67%		1.57%
Over 30 days to 90 days
Linked Repurchase Agreements
Balance	$ 10,900		$ 21,599
Weighted Average Interest Rate (as a percent)	1.45%		1.66%