Consolidated Statement Of Assets, Liabilities And Shareholders' Equity (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statement Of Assets, Liabilities And Shareholders' Equity [Abstract]
Investments at value, cost	$ 1,234,203	$ 1,232,484
Financial derivatives - assets, cost	118,281	208,958
Repurchase agreements, cost	15,750	25,684
Investments sold short at fair value, proceeds	459,013	775,782
Financial derivatives - liabilities, proceeds	$ 9,636	$ 17,718
Common stock, no par value
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	16,447,651	16,498,342
Common stock, shares outstanding	16,447,651	16,498,342