Fair value measurement - Assets and Liabilities (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 143,322	$ 161,851
Total assets	143,322	161,851
Liabilities, Fair Value Disclosure [Abstract]
Contingent consideration	4,020	3,964
Total liabilities	4,994	5,599
Related party
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	504	1,298
Debt
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	470	337
Fair value hierarchy Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	143,322	161,851
Total assets	143,322	161,851
Level 3
Liabilities, Fair Value Disclosure [Abstract]
Contingent consideration	4,020	3,964
Total liabilities	4,994	5,599
Level 3 | Related party
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	504	1,298
Level 3 | Debt
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	$ 470	$ 337