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Fair value measurement - Assets and liabilities measured on a recurring basis (Details) - Recurring - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]    
Cash and cash equivalents $ 211,717 $ 237,342
Total assets 211,717 237,342
Liabilities, Fair Value Disclosure [Abstract]    
Total liabilities 2,603 1,375
Related party    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative financial instruments 2,603 803
Debt    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative financial instruments   572
Fair value hierarchy Level 1    
Assets, Fair Value Disclosure [Abstract]    
Cash and cash equivalents 211,717 237,342
Total assets 211,717 237,342
Level 3    
Liabilities, Fair Value Disclosure [Abstract]    
Total liabilities 2,603 1,375
Level 3 | Related party    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative financial instruments $ 2,603 803
Level 3 | Debt    
Liabilities, Fair Value Disclosure [Abstract]    
Derivative financial instruments   $ 572