Stockholders' equity - Summary of the Company's Stock Options Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options, Granted	0	0
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options, Options outstanding, beginning of period	3,365,692
Number of options, Granted
Number of options, Exercised	(1,045,644)
Number of options, Expired / forfeited	(50,134)
Number of options, Options outstanding, end of period	2,269,914
Number of options, Options exercisable, end of period	2,227,102
Number of options, Options expected to vest, end of period	30,076
Weighted average exercise price, Options outstanding, beginning of period	$ 6.15
Weighted average exercise price, Granted
Weighted average exercise price, Exercised	$ 7.05
Weighted average exercise price, Expired / forfeited	$ 14.97
Weighted average exercise price, Options outstanding, end of period	$ 5.65
Weighted average exercise price, Options exercisable, end of period	$ 5.67
Weighted average exercise price, Options expected to vest, end of period	$ 4.71
Weighted Average Remaining Contractual Life (years), Options outstanding, end of period	4 years 8 months 19 days
Weighted Average Remaining Contractual Life (years), Options exercisable, end of period	4 years 8 months 16 days
Weighted Average Remaining Contractual Life (years), Options expected to vest, end of period	5 years 4 months 6 days
Aggregate intrinsic value, Granted
Aggregate intrinsic value, Exercised
Aggregate intrinsic value, Expired / forfeited
Aggregate intrinsic value, Options outstanding, end of period	17,563
Aggregate intrinsic value, Options exercisable, end of period	17,196
Aggregate intrinsic value, Options expected to vest, end of period	$ 259