Earnings per share - Computation of Basic (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic earnings (loss) per share:
Net income (loss)	$ 7,288	$ 3,776	$ 11,433	$ (3,138)
Redeemable convertible preferred stock dividends declared	(2,936)	(2,936)	(5,872)	(5,872)
Deemed dividend resulting from modification of redeemable convertible preferred stock	(2,929)	0	(2,929)	0
Net income (loss) attributed to common stockholders	$ 1,423	$ 840	$ 2,632	$ (9,010)
Weighted average common shares outstanding (shares)	7,049,863	7,505,505	7,043,571	7,451,737
Basic earnings (loss) per share (usd per share)	$ 0.20	$ 0.11	$ 0.37	$ (1.21)