Fair value measurement - Assets and liabilities measured on a recurring basis (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 344,950	$ 380,726
Total assets	344,950	380,726
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	1,004	3,075
Related party
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	1,004	3,075
Fair value hierarchy Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	344,950	380,726
Total assets	344,950	380,726
Level 3
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	1,004	3,075
Level 3 | Related party
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	$ 1,004	$ 3,075