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Stock award plan (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Mar. 05, 2013
May. 31, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Stock option plan          
Additional compensation recognized due to modifications to the options granted       $ 1,900 $ 600
Stock option          
Number of options          
Outstanding at the beginning of the period (in shares)     3,432,972 2,095,592 42,394
Granted (in shares)     2,201,800 1,639,996 2,117,113
Exercised (in shares)     (656,248) (108,645)  
Forfeited (in shares)     (152,047) (193,971) (63,915)
Outstanding at the end of the period (in shares)     4,826,477 3,432,972 2,095,592
Vested or expected to vest at the end of the period (in shares)     3,266,769    
Exercisable at the end of the period (in shares)     1,357,478    
Weighted-average exercise price          
Outstanding at the beginning of the period (in dollars per share)     $ 25.00 $ 20.24 $ 474.00
Granted (in dollars per share)     50.81 31.09 11.29
Exercised (in dollars per share)     13.27 11.00  
Forfeited (in dollars per share)     49.58 32.91 12.57
Outstanding at the end of the period (in dollars per share)     37.20 $ 25.00 $ 20.24
Vested or expected to vest at the end of the period (in dollars per share)     40.14    
Exercisable at the end of the period (in dollars per share)     $ 29.38    
Weighted-average remaining contractual term          
Outstanding at the end of the period     8 years 4 months 17 days    
Vested or expected to vest at the end of the period     8 years 7 months 24 days    
Exercisable at the end of the period     7 years 8 months 1 day    
Aggregate intrinsic value          
Outstanding at the end of the period (in dollars)     $ 32,391    
Vested or expected to vest at the end of the period (in dollars)     13,590    
Exercisable at the end of the period (in dollars)     $ 18,242    
Valuation assumptions          
Expected dividend yield (as a percent)     0.00% 0.00% 0.00%
Weighted average grant date fair value (in dollars per share)     $ 50.81 $ 22.39 $ 8.23
Stock option | Minimum          
Stock option plan          
Vesting period     3 years    
Valuation assumptions          
Risk-free interest rate (as a percent)     1.48% 0.11% 0.85%
Expected volatility (as a percent)     67.00% 70.00% 87.00%
Expected term     5 years 6 months 5 years 6 months 6 years
Stock option | Maximum          
Stock option plan          
Vesting period     4 years    
Valuation assumptions          
Risk-free interest rate (as a percent)     2.18% 2.04% 1.90%
Expected volatility (as a percent)     69.00% 91.00% 89.00%
Expected term     9 years 1 month 13 days 6 years 3 months 6 years 3 months
2009 Equity and Long Term Incentive Plan          
Stock option plan          
Number of shares available for issuance   0      
Number of additional shares authorized   2,500,000      
2009 Equity and Long Term Incentive Plan | Stock option | Minimum          
Stock option plan          
Stock options granted, exercise price as percentage of the fair market value of common stock at grant date     100.00%    
Stock options granted to stockholder with specified ownership percentage, exercise price as percentage of the fair market value of common stock at grant date     110.00%    
Stockholder's specified ownership percentage     10.00%    
2009 Equity and Long Term Incentive Plan | Stock option | Maximum          
Stock option plan          
Expiration period     10 years    
2013 Stock Incentive Plan          
Stock option plan          
Number of shares available for issuance 0        
2013 Stock Incentive Plan | Restricted stock          
Stock option plan          
Options granted (in shares) 735,324        
2013 Stock Incentive Plan | Stock option          
Number of options          
Granted (in shares) 4,613        
2013 Stock Incentive Plan | Common stock          
Stock option plan          
Number of shares authorized 739,937        
2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan | Common stock          
Stock option plan          
Number of shares available for issuance   122,296      
2013 Long Term Incentive Plan | Common stock          
Stock option plan          
Number of shares available for issuance     266,533    
Annual increase in the number of shares outstanding on the first day of the fiscal year (as a percent)   4.00%      
2013 Long Term Incentive Plan | Common stock | Minimum          
Stock option plan          
Annual increase in the number of shares on the first day of the fiscal year   2,500,000      
2013 Long Term Incentive Plan | Common stock | Maximum          
Stock option plan          
Number of shares subject to outstanding awards   3,040,444      
Inducement grant plan | Stock option          
Number of options          
Granted (in shares)     806,600    
Forfeited (in shares)     (84,150)    
Exercise price          
Granted (in dollars per share)     $ 50.46