Fair value measurement - Assets and liabilities measured on a recurring basis (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 316,948	$ 380,726
Total assets	316,948	380,726
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	832	3,075
Related party
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	832	3,075
Fair value hierarchy Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	316,948	380,726
Total assets	316,948	380,726
Level 3
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	832	3,075
Level 3 | Related party
Liabilities, Fair Value Disclosure [Abstract]
Derivative financial instruments	$ 832	$ 3,075