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Fair value (Details) - Fair value measurements on a recurring basis - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Assets:    
Total assets $ 147,653 $ 126,662
Level 1    
Assets:    
Total assets 95,894 43,224
Level 2    
Assets:    
Total assets 51,759 83,438
Cash and money market funds    
Assets:    
Total assets 95,894 43,224
Cash and money market funds | Level 1    
Assets:    
Total assets 95,894 43,224
Corporate debt securities (including commercial paper)    
Assets:    
Total assets 51,759 83,438
Corporate debt securities (including commercial paper) | Level 2    
Assets:    
Total assets $ 51,759 $ 83,438