SELECTED QUARTERLY FINANCIAL DATA (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
SELECTED QUARTERLY FINANCIAL DATA [Abstract]
Net investment income (loss)	$ (2,133)	$ 4,142	$ (120)	$ 18	$ (1,878)	$ 4,385	$ 23	$ 14	$ 1,907	$ 2,544	$ 1,804
Net realized gain (loss) on investments	20,912	640	(7)	66,888	349	(2,360)	18,500	(5,911)	88,433	10,578	38,885
Net increase (decrease) in unrealized appreciation of investments	22,271	22,296	50,321	(78,521)	78,501	48,798	(44,076)	(4,588)	16,367	78,635	12,999
Increase in net assets from operations	$ 41,050	$ 27,078	$ 50,194	$ (11,615)	$ 77,092	$ 50,823	$ (25,703)	$ (10,455)	$ 106,707	$ 91,757	$ 53,688
Net increase (decrease) in net operations per share (in dollars per share)	$ 10.77	$ 7.11	$ 13.18	$ (3.05)	$ 20.53	$ 13.53	$ (6.85)	$ (2.79)	$ 28.01	$ 24.42