Lease costs (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 10,387	$ 21,908
Variable lease cost	400	799
Short-term lease cost	40,474	57,712
Total lease costs	$ 51,261	$ 80,419
Weighted average remaining lease term	3 years 7 days	3 years 7 days
Weighted average discount rate	4.46%	4.46%
Cash paid for amounts included in the measurement of lease liabilities		$ 21,798