Earnings per share - Computation of Diluted (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		Dec. 31, 2011
Earnings Per Share, Diluted [Abstract]
Net income (loss) attributable to common stockholders—Diluted	$ 7,288		$ 3,776		$ 11,433		$ (9,010)
Weighted average common shares outstanding (shares)	7,049,863		7,505,505		7,043,571		7,451,737
Options to purchase common stock (shares)	4,602,055		3,082,612		3,954,035		0
Convertible preferred stock (as converted basis) (shares)	26,767,656		26,767,656		26,767,656		0
Contingent preferred stock per Election and Amendment Agreement (shares)	151,111	[1]	0	[1]	151,111	[1]	0	[1]
Convertible preferred stock warrants (as converted basis) (shares)	50,009		35,757		42,996		0
Weighted average shares and potential diluted shares (shares)	38,620,694		37,391,530		37,959,369		7,451,737
Diluted earnings (loss) per share (usd per share)	$ 0.19		$ 0.10		$ 0.30		$ (1.21)
Common stock, fair value (usd per share)	$ 26.50				$ 26.50				$ 20.14

[1] Additional preferred stock per Election and Amendment Agreement as discussed in “-Note 11-Redeemable Convertible Preferred Stock." Shares calculated based on common stock fair value of $26.50 as of June 30, 2012.