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Stock award plan - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Mar. 05, 2013
Jun. 30, 2016
May 31, 2016
May 31, 2013
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock option plan              
Additional compensation recognized due to modifications to the options granted             $ 1,900
Share-based compensation expense         $ 30,559 $ 35,009 $ 33,979
Unrecognized compensation cost         $ 38,200    
Weighted average remaining service period for recognition of unrecognized compensation cost         1 year 10 months 28 days    
Unvested restricted stock              
Stock option plan              
Options granted (in shares)         365,194    
Stock option              
Stock option plan              
Granted (in shares)         1,913,873 1,500,645 2,201,800
Forfeited (in shares)         1,117,462 383,590 152,047
Expected dividend yield         0.00% 0.00% 0.00%
Weighted average grant date fair value (in dollars per share)         $ 8.45 $ 17.31 $ 50.81
Stock option | Maximum              
Stock option plan              
Vesting period         4 years    
Stock option | Minimum              
Stock option plan              
Vesting period         3 years    
Stock Appreciation Rights (SARs)              
Stock option plan              
Granted (in shares)     897,290        
Vesting period     4 years        
Share-based compensation expense         $ 1,900    
Employee Stock Purchase Plan              
Stock option plan              
Number of shares authorized (in shares)         1,000,000    
Award requisite service period         6 months    
Purchase price of common stock         85.00%    
Employee stock purchase plan, voting percentage limit   5.00%          
Share-based compensation expense         $ 900    
2013 Stock Incentive Plan              
Stock option plan              
Number of shares available for issuance (in shares) 0            
2013 Stock Incentive Plan | Common stock              
Stock option plan              
Number of shares authorized (in shares) 739,937            
2013 Stock Incentive Plan | Unvested restricted stock              
Stock option plan              
Options granted (in shares) 735,324            
2013 Stock Incentive Plan | Stock option              
Stock option plan              
Granted (in shares) 4,613            
2009 Equity and Long Term Incentive Plan              
Stock option plan              
Number of shares available for issuance (in shares)       0      
Number of additional shares authorized (in shares)       2,500,000      
2009 Equity and Long Term Incentive Plan | Stock option              
Stock option plan              
Stockholder's specified ownership percentage         10.00%    
Expiration period         10 years    
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%    
2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan | Common stock              
Stock option plan              
Number of shares available for issuance (in shares)       122,296      
2013 Long Term Incentive Plan | Common stock              
Stock option plan              
Number of shares available for issuance (in shares)         927,478    
Number of shares subject to outstanding awards (in shares)       3,040,444      
Annual increase in the number of shares on the first day of the fiscal year (in shares)       2,500,000      
Annual increase in the number of shares outstanding on the first day of the fiscal year       4.00%      
Inducement grant plan | Stock option              
Stock option plan              
Granted (in shares)         640,550    
Granted (in dollars per share)         $ 14.16    
Forfeited (in shares)         316,176