Investments in joint ventures (Details) (USD $)	12 Months Ended							12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2010 C-BASS [Member]	Dec. 31, 2009 C-BASS [Member]	Dec. 31, 2007 C-BASS [Member]	Dec. 31, 2010 Sherman [Member]	Dec. 31, 2008 Sherman [Member]	Jul. 31, 2008 Sherman [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, approximate ownership percentage					less than 50%
Carrying value of investment in joint venture					$ 0	$ 0	$ 0
Equity method investment, current book value					0	0	0
Equity method investment, ownership percentage (in hundredths)										24.25%
Cash proceeds from sale of investment in joint venture									124,500,000
Unsecured promissory note received as consideration in sale of investment in joint venture									85,000,000
Scheduled maturity of unsecured promissory note				Feb. 13, 2011
Variable rate basis of unsecured promissory note									three-month LIBOR
Basis spread on variable rate of unsecured promissory note (in basis points)									500
Proceeds from unsecured promissory note	0	83,500,000	0					83,500,000
Carrying value of unsecured promissory note								84,000,000
Loss recognized on unsecured promissory note	$ 0	$ (466,000)	$ 0					$ 500,000