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Stock award plan (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 9 Months Ended
Mar. 05, 2013
May. 31, 2013
Sep. 30, 2015
Stock option      
Number of options      
Outstanding at the beginning of the period (in shares)     3,432,972
Granted (in shares)     2,086,300
Exercised (in shares)     (654,748)
Forfeited (in shares)     (74,366)
Expired (in shares)     (1,894)
Outstanding at the end of the period (in shares)     4,788,264
Vested or expected to vest at the end of the period (in shares)     4,497,155
Exercisable at the end of the period (in shares)     1,132,683
Weighted-average exercise price      
Outstanding at the beginning of the period (in dollars per share)     $ 25.00
Granted (in dollars per share)     52.04
Exercised (in dollars per share)     13.27
Forfeited (in dollars per share)     42.46
Expired (in dollars per share)     285.48
Outstanding at the end of the period (in dollars per share)     37.64
Vested or expected to vest at the end of the period (in dollars per share)     37.00
Exercisable at the end of the period (in dollars per share)     $ 32.21
Weighted-average remaining contractual term      
Outstanding at the end of the period     8 years 6 months 29 days
Vested or expected to vest at the end of the period     8 years 6 months 22 days
Exercisable at the end of the period     7 years 10 months 21 days
Aggregate intrinsic value      
Outstanding at the end of the period (in dollars)     $ 20,137
Vested or expected to vest at the end of the period (in dollars)     19,018
Exercisable at the end of the period (in dollars)     $ 10,312
Valuation assumptions      
Inducement grants for non-statutory stock options     691,100
Expected dividend yield (as a percent)     0.00%
Weighted average grant date fair value (in dollars per share)     $ 32.32
Stock option | Minimum      
Valuation assumptions      
Risk-free interest rate (as a percent)     1.49%
Expected volatility (as a percent)     67.00%
Expected term     5 years 6 months
Stock option | Maximum      
Valuation assumptions      
Risk-free interest rate (as a percent)     2.01%
Expected volatility (as a percent)     69.00%
Expected term     6 years 1 month 10 days
2013 Stock Incentive Plan      
Stock option plan      
Number of shares available for issuance 0    
2013 Stock Incentive Plan | Unvested restricted stock      
Stock option plan      
Number of shares of restricted stock granted 735,324    
2013 Stock Incentive Plan | Stock option      
Stock option plan      
Number of shares of restricted stock granted 4,613    
2013 Stock Incentive Plan | Common stock      
Stock option plan      
Number of shares authorized 739,937    
2009 Equity and Long Term Incentive Plan      
Stock option plan      
Number of additional shares authorized   2,500,000  
2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan | Common stock      
Stock option plan      
Number of shares available for issuance   238,427  
2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan | Common stock | Maximum      
Stock option plan      
Number of shares subject to outstanding awards   3,040,444  
2013 Long Term Incentive Plan | Minimum      
Stock option plan      
Annual increase in the number of shares on the first day of the fiscal year   2,500,000  
Annual increase in the number of shares outstanding on the first day of the fiscal year (as a percent)   4.00%