Stockholders' equity (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Preferred stock
Preferred stock, shares authorized (in shares)	15,000,000		15,000,000
Preferred stock, par value (in dollars per share)	$ 0.001		$ 0.001
Common stock
Common stock, par value (in dollars per share)	$ 0.001		$ 0.001
Common stock, shares authorized (in shares)	100,000,000		100,000,000
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,976,000	$ 2,712,000
Stock Options [Member]
Aggregate Intrinsic Value [Abstract]
Outstanding, beginning of period			4,801,378
Outstanding, end of period	2,848,569		4,801,378
Exercisable, end of period	2,848,569
Restricted Stock Units (RSUs) [Member]
Restricted stock unit award activity [Roll Forward]
Outstanding, beginning of period (in shares)	715,609
Granted (in shares)	406,454
Vested (in shares)	(291,619)
Forfeited (in shares)	(15,848)
Outstanding, end of period (in shares)	814,596
Weighted-Average Grant Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 18.41
Granted (in dollars per share)	$ 14.67
Vested (in dollars per share)	$ 14.77
Forfeited (in dollars per share)	$ 17.16
Outstanding, end of period (in dollars per share)	$ 16.46
Aggregate intrinsic value [Abstract]
Outstanding, beginning of period	11,478,368
Outstanding, end of period	$ 5,737,280
2006 Plan [Member] | Stock Options [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	3,550,842
Granted (in shares)	812,910
Exercised (in shares)	(56,499)
Forfeited (in shares)	(127,786)
Outstanding, end of period (in shares)	4,179,467
Exercisable, end of period (in shares)	2,575,695
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 17.07
Granted (in dollars per share)	$ 14.67
Exercised (in dollars per share)	$ 8.91
Forfeited (in dollars per share)	$ 18.55
Outstanding, end of period (in dollars per share)	$ 16.67
Exercisable, end of period (in dollars per share)	$ 16.86
2004 Plan [Member] | Stock Options [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	53,156
Granted (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Outstanding, end of period (in shares)	53,156
Exercisable, end of period (in shares)	53,156
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 8.86
Granted (in dollars per share)	$ 0
Exercised (in dollars per share)	$ 0
Forfeited (in dollars per share)	$ 0
Outstanding, end of period (in dollars per share)	$ 8.86
Exercisable, end of period (in dollars per share)	$ 8.86