Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Investments at value, cost	$ 2,694,223	$ 2,071,754
Financial derivatives–assets, at fair value Net cost	20,895	31,474
Repurchase agreements, cost	160,468	155,109
Investments sold short at value, proceeds	(697,686)	(640,202)
Financial derivatives-liabilities, at fair value Net proceeds	(16,294)	(27,463)
Other secured borrowings proceeds	(114,190)	(57,909)
Other secured financing, at fair value-Proceeds	$ (90,409)	$ (125,105)
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	30,155,055	31,335,938
Common stock, shares outstanding	30,155,055	31,335,938