Lease costs (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 10,536	$ 32,444
Variable lease cost	381	1,180
Short-term lease cost	28,270	85,982
Total lease costs	$ 39,187	$ 119,606
Weighted average remaining lease term	2 years 11 months 23 days	2 years 11 months 23 days
Weighted average discount rate	4.63%	4.63%
Cash paid for amounts included in the measurement of lease liabilities		$ 32,261