Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Investments at value, cost	$ 2,071,754	$ 1,525,710
Financial derivatives–assets, at fair value Net cost	31,474	40,724
Repurchase agreements, cost	155,109	185,205
Investments sold short at value, proceeds	(640,202)	(589,429)
Financial derivatives-liabilities, at fair value Net proceeds	(27,463)	(12,012)
Other secured borrowings proceeds	(57,909)	(24,086)
Other secured financing, at fair value-Proceeds	$ (125,105)	$ 0
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	31,335,938	32,294,703
Common stock, shares outstanding	31,335,938	32,294,703