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Income taxes - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating loss carryforwards      
Valuation allowance $ 177,631 $ 183,015  
Valuation allowance, deferred tax asset, increase (decrease) 5,400 $ 43,400  
Orphan drug credit carryover 67,800    
Limited NOL carryforwards 231,500    
Operating loss carryforwards, not subject to expiration 169,200    
Operating loss carryforwards, subject to expiration 62,300    
Operating loss carryforwards for immediate use 196,700    
Operating loss carryforwards in next twelve months $ 11,200    
Federal income tax at statutory rate 34.00% 34.00% 34.00%
Uncertain tax position $ 0    
Change In tax rate, deferred tax asset, provisional income tax expense 46,100    
First five years from ownership change      
Operating loss carryforwards      
Net operating loss carryforwards 16,700    
After five years      
Operating loss carryforwards      
Net operating loss carryforwards 5,700    
Federal      
Operating loss carryforwards      
Net operating loss carryforwards 290,800    
Research and development credit carryforwards 13,200    
State      
Operating loss carryforwards      
Net operating loss carryforwards 167,100    
Research and development credit carryforwards 8,700    
Accounting Standards Update 2016-09      
Operating loss carryforwards      
Share-based compensation cost $ 50,400