Property and equipment, net - Lease and maturity of lease liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Lease liabilities
Lessee, Operating Lease, Existence of Residual Value Guarantee [true false]			false
Lessee, Operating Lease, Existence of Option to Extend [true false]			true
Operating lease liabilities, current	$ 10,059		$ 10,059		$ 9,343
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Accrued Liabilities, Current		Accrued Liabilities, Current		Accrued Liabilities, Current
Finance lease liabilities, current	$ 1,189		$ 1,189		$ 664
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	Finance lease liabilities, current		Finance lease liabilities, current		Finance lease liabilities, current
Operating lease liabilities, noncurrent	$ 13,956		$ 13,956		$ 11,854
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other Liabilities, Noncurrent		Other Liabilities, Noncurrent		Other Liabilities, Noncurrent
Finance lease liabilities, noncurrent	$ 31,917		$ 31,917		$ 31,918
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Finance lease liabilities, noncurrent		Finance lease liabilities, noncurrent		Finance lease liabilities, noncurrent
Total lease liabilities	$ 57,121		$ 57,121		$ 53,779
Cash paid for operating lease liabilities			5,900	$ 5,400
Cash paid for financing lease liabilities in financing cash flows			$ 387	409
Maturity of our finance lease liabilities
Weighted average lease term, finance leases	15 years 2 months 12 days		15 years 2 months 12 days
Weighted average lease term, operating leases	5 years 3 months 18 days		5 years 3 months 18 days
Weighted average discount rate, finance leases	3.60%		3.60%
Weighted average discount rate, operating leases	4.40%		4.40%
Expense related to our operating leases	$ 2,900	$ 3,600	$ 6,100	7,300
Amortization on our finance lease right-of-use assets	700	200	1,300	400
Interest expense on finance lease liabilities	300		600
Rent expense	$ 2,900	$ 3,700	$ 6,400	$ 7,000