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Stockholders' equity - Summary of Restricted Stock Unit Activity (Detail) (Restricted Stock [Member], USD $)
In Thousands, except Share data, unless otherwise specified
6 Months Ended
Jun. 30, 2014
Restricted Stock [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of Units outstanding, beginning of period 1,279,123
Number of Units, Granted 463,196
Number of Units, Released (348,171)
Number of Units, Forfeited (135,375)
Number of Units outstanding, end of period 1,258,773
Weighted Average Grant Date Fair Value, Units outstanding, beginning of period $ 9.67
Weighted Average Grant Date Fair Value, Granted $ 16.85
Weighted Average Grant Date Fair Value, Released $ 7.43
Weighted Average Grant Date Fair Value, Forfeited $ 12.99
Weighted Average Grant Date Fair Value, Units outstanding, end of period $ 12.59
Aggregate intrinsic value, Granted   
Aggregate intrinsic value, Released   
Aggregate intrinsic value, Forfeited   
Aggregate intrinsic value, Units outstanding, end of period $ 16,742