Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Investments at value, cost	$ 1,758,854	$ 1,525,710
Financial derivatives–assets, at fair value Net cost	41,041	40,724
Repurchase agreements, cost	194,265	185,205
Investments sold short at value, proceeds	672,506	589,429
Financial derivatives-liabilities, at fair value Net proceeds	28,507	12,012
Other secured borrowings proceeds	$ 89,646	$ 24,086
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	31,992,177	32,294,703
Common stock, shares outstanding	31,992,177	32,294,703