Stockholders' equity (Details) (USD $)	6 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Stock Options [Member]	Dec. 31, 2011 Stock Options [Member]	Jun. 30, 2012 2006 Plan [Member] Stock Options [Member]	Jun. 30, 2012 2006 Plan [Member] Restricted Stock Units (RSUs) [Member]	Jun. 30, 2012 2004 Plan [Member] Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock based employee compensation plans	2
Common stock authorized for issuance under the plan (in shares)	11,178,826
Common stock available for future awards (in shares)	3,757,296
Restricted stock unit conversion ratio	1.86
Maximum number of shares that may be granted per year to a single participant (in shares)	287,700
Share Based Compensation Arrangement by share Based Payment Award, Contractual life of awards	10 years
Options outstanding [Roll Forward]
Outstanding beginning of period				3,090,909		53,156
Granted (in shares)				726,291		0
Exercised (in shares)				(43,491)		0
Forfeited (in shares)				(125,678)		0
Outstanding end of period				3,648,031		53,156
Exercisable end of period				2,176,650		53,156
Weighted-Average Exercise Price [Roll Forward]
Outstanding Beginning of period (in dollars per share)				$ 17.35		$ 8.86
Granted (in dollars per share)				$ 15.75		$ 0
Exercised (in dollars per share)				$ 9.23		$ 0
Forfeited (in dollars per share)				$ 19.52		$ 0
Outstanding End of period (in dollars per share)				$ 17.06		$ 8.86
Exercisable End of period (in dollars per share)				$ 16.09		$ 8.86
Aggregate Intrinsic Value [Abstract]
Outstanding Beginning of period		$ 4,227,300	$ 6,238,427
Outstanding End of period		4,227,300	6,238,427
Exercisable End of period		4,200,518
Restricted stock unit award activity [Roll Forward]
Outstanding beginning of period (in shares)					635,500
Granted (in shares)					363,197
Vested (in shares)					(228,369)
Forfeited (in shares)					(33,545)
Outstanding end of period (in shares)					736,783
Weighted-Average Grant Price [Roll Forward]
Outstanding beginning of period (in dollars per share)					$ 20.89
Granted (in dollars per share)					$ 15.75
Vested (in dollars per share)					$ 20.26
Forfeited (in dollars per share)					$ 20.10
Outstanding end of period (in dollars per share)					$ 18.58
Aggregate intrinsic value [Abstract]
Beginning of period					10,714,450
End of period					$ 11,162,262