Stockholders' equity (Details) (USD $)	1 Months Ended	9 Months Ended			9 Months Ended
	Oct. 31, 2012	Sep. 30, 2012	Sep. 30, 2012 Stock Options [Member]	Dec. 31, 2011 Stock Options [Member]	Sep. 30, 2012 Restricted Stock Units (RSUs) [Member]	Sep. 30, 2012 2006 Plan [Member] Stock Options [Member]	Sep. 30, 2012 2004 Plan [Member] Stock Options [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount for the repurchase program		$ 35,000,000
Number of shares repurchased	301,000	98,000
Amount of treasury stock acquired	4,400,000	1,500,000
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)		3,757,713
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)						743,491	0
Exercised (in shares)						(56,891)	0
Forfeited (in shares)						(163,484)	0
Outstanding end of period						3,614,025	53,156
Exercisable end of period						2,177,151	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)						$ 15.72	$ 0
Exercised (in dollars per share)						$ 8.71	$ 0
Forfeited (in dollars per share)						$ 19.38	$ 0
Outstanding End of period (in dollars per share)						$ 17.06	$ 8.86
Exercisable End of period (in dollars per share)						$ 16.15	$ 8.86
Aggregate Intrinsic Value [Abstract]
Outstanding Beginning of period			3,480,855	6,238,427
Outstanding End of period			3,480,855	6,238,427
Exercisable End of period			3,480,855
Restricted stock unit award activity [Roll Forward]
Outstanding beginning of period (in shares)					635,500
Granted (in shares)					391,797
Vested (in shares)					(256,802)
Forfeited (in shares)					(40,763)
Outstanding end of period (in shares)					729,732
Weighted-Average Grant Price [Roll Forward]
Outstanding beginning of period (in dollars per share)					$ 20.89
Granted (in dollars per share)					$ 15.67
Vested (in dollars per share)					$ 15.23
Forfeited (in dollars per share)					$ 19.88
Outstanding end of period (in dollars per share)					$ 18.53
Aggregate intrinsic value [Abstract]
Beginning of period					10,714,450
End of period					$ 10,369,492