Consolidated Statement Of Assets, Liabilities And Equity (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Investments at value, cost	$ 1,465,788	$ 1,328,153
Financial derivatives - assets, cost	56,769	65,860
Repurchase agreements, cost	42,614	13,650
Investments sold short at value, proceeds	(648,101)	(621,048)
Financial derivatives - liabilities, proceeds	(14,358)	(13,171)
Securitized debt proceeds	$ (1,250)	$ (1,311)
Common stock, par value
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	20,403,723	20,370,469
Common stock, shares outstanding	20,403,723	20,370,469