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Stock award plan (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Mar. 05, 2013
May. 31, 2013
Mar. 31, 2016
Dec. 31, 2015
Unvested restricted stock awards and units        
Number of options        
Granted (in shares)     141,185  
Stock option        
Number of options        
Outstanding at the beginning of the period (in shares)     4,826,477  
Granted (in shares)     1,357,045  
Exercised (in shares)     (3,125)  
Forfeited/Cancelled (in shares)     (145,479)  
Outstanding at the end of the period (in shares)     6,034,918 4,826,477
Vested or expected to vest at the end of the period (in shares)     3,716,134  
Exercisable at the end of the period (in shares)     2,026,638  
Weighted-average exercise price        
Outstanding at the beginning of the period (in dollars per share)     $ 37.20  
Granted (in dollars per share)     29.78  
Exercised (in dollars per share)     10.85  
Forfeited (in dollars per share)     49.51  
Outstanding at the end of the period (in dollars per share)     35.38 $ 37.20
Vested or expected to vest at the end of the period (in dollars per share)     36.19  
Exercisable at the end of the period (in dollars per share)     $ 33.63  
Weighted-average remaining contractual term        
Outstanding at the end of the period     8 years 5 months 19 days 8 years 4 months 17 days
Vested or expected to vest at the end of the period     8 years 10 months 6 days  
Exercisable at the end of the period     7 years 8 months 9 days  
Aggregate intrinsic value        
Outstanding at the end of the period (in dollars)     $ 1 $ 32,391
Vested or expected to vest at the end of the period (in dollars)     $ 1  
Valuation assumptions        
Inducement grants for non-statutory stock options     67,100  
Expected dividend yield (as a percent)     0.00%  
Weighted average grant date fair value (in dollars per share)     $ 18.42  
Stock option | Minimum        
Valuation assumptions        
Risk-free interest rate (as a percent)     1.32%  
Expected volatility (as a percent)     67.00%  
Expected term     5 years 18 days  
Stock option | Maximum        
Valuation assumptions        
Risk-free interest rate (as a percent)     2.24%  
Expected volatility (as a percent)     70.00%  
Expected term     10 years  
2009 Equity and Long Term Incentive Plan        
Stock option plan        
Number of additional shares authorized   2,500,000    
2013 Stock Incentive Plan        
Stock option plan        
Number of shares available for issuance 0      
2013 Stock Incentive Plan | Unvested restricted stock awards and units        
Stock option plan        
Number of shares of restricted stock granted 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    
2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan | Common stock | 1998 Employee, Director and Consultant Stock Option Plan | 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%    
2013 Long Term Incentive Plan | Common stock        
Stock option plan        
Number of shares available for issuance     290,868