Right-of-use asset and lease liabilities - Operating lease liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018
Components of lease cost
Operating lease cost	$ 897
Variable lease cost	4
Sublease income	(268)
Total lease cost	633	$ 700
Assets
Operating lease right-of-use asset	18,678
Current
Current operating lease liability	3,808
Non-current
Non-current operating lease liability	23,344
Total lease liabilities	$ 27,152
Weighted-average remaining lease term, Operating leases	11 years 6 months
Weighted-average discount rate, Operating leases	11.79%
Supplemental cash flow information related to leases
Cash paid for amounts included in the measurement of lease liabilities, Operating cash flows for operating leases	$ 1,413
Undiscounted Cash Flows
2019 (nine months remaining)	2,855
2020	3,848
2021	3,901
2022	3,955
2023	4,008
Thereafter	30,891
Total minimum lease payments	49,458
Less: amount of lease payments representing interest	(22,306)
Total lease liabilities	27,152
Less: current obligations under operating leases	(3,808)
Non-current operating lease liability	23,344
ASU 2016-02
Non-current
Total lease liabilities			$ 28,100
Undiscounted Cash Flows
2019 (nine months remaining)				$ 4,670
2020				5,330
2021				5,425
2022				5,522
2023				5,620
Thereafter				40,977
Total minimum lease payments				67,544
Total lease liabilities			$ 28,100
Lexington
Current
Current operating lease liability	1,916
Non-current
Non-current operating lease liability	12,477
Total lease liabilities	14,393
Undiscounted Cash Flows
2019 (nine months remaining)	1,436
2020	1,956
2021	2,009
2022	2,063
2023	2,116
Thereafter	15,754
Total minimum lease payments	25,334
Less: amount of lease payments representing interest	(10,941)
Total lease liabilities	14,393
Less: current obligations under operating leases	(1,916)
Non-current operating lease liability	12,477
Lexington | ASU 2016-02
Undiscounted Cash Flows
2019 (nine months remaining)				2,707
2020				3,360
2021				3,455
2022				3,552
2023				3,650
Thereafter				24,892
Total minimum lease payments				41,616
Amsterdam
Current
Current operating lease liability	1,892
Non-current
Non-current operating lease liability	10,867
Total lease liabilities	12,759
Undiscounted Cash Flows
2019 (nine months remaining)	1,419
2020	1,892
2021	1,892
2022	1,892
2023	1,892
Thereafter	15,137
Total minimum lease payments	24,124
Less: amount of lease payments representing interest	(11,365)
Total lease liabilities	12,759
Less: current obligations under operating leases	(1,892)
Non-current operating lease liability	$ 10,867
Amsterdam | ASU 2016-02
Undiscounted Cash Flows
2019 (nine months remaining)				1,963
2020				1,970
2021				1,970
2022				1,970
2023				1,970
Thereafter				16,085
Total minimum lease payments				$ 25,926