Right-of-use asset and lease liabilities - Operating lease liabilities (Details) $ in Thousands	1 Months Ended			3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) floor $ / €	Feb. 29, 2020 floor	Dec. 31, 2017 floor	Mar. 31, 2020 USD ($) $ / €	Mar. 31, 2019 USD ($)	Dec. 31, 2017 floor	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Components of lease cost
Operating lease cost				$ 1,290	$ 897
Variable lease cost				145	4
Sublease income				(248)	(268)
Total lease cost				1,187	633
Assets
Operating lease right-of-use assets	$ 26,288			26,288			$ 26,797	$ 19,000
Current liabilities
Current operating lease liability	5,900			5,900			5,865
Non-current liabilities
Non-current operating lease liability	30,518			30,518			31,133
Total lease liabilities	$ 36,418			$ 36,418			$ 36,998
Weighted-average remaining lease term, Operating leases	10 years 1 month 6 days			10 years 1 month 6 days			10 years 3 months 18 days
Weighted-average discount rate, Operating leases	11.33%			11.33%			11.33%
Supplemental cash flow information related to leases
Cash paid for amounts included in the measurement of lease liabilities, Operating cash flows for operating leases				$ 1,424	$ 1,413
Undiscounted Cash Flows
2020 (nine months remaining)	$ 4,529			4,529
2021	5,449			5,449
2022	5,407			5,407
2023	5,505			5,505
2024	6,001			6,001
Thereafter	33,578			33,578
Total minimum lease payments	60,469			60,469
Less: amount of lease payments representing interest	(24,051)			(24,051)
Total lease liabilities	36,418			36,418			$ 36,998
Less: current obligations under operating leases	(5,900)			(5,900)			(5,865)
Non-current operating lease liability	$ 30,518			$ 30,518			$ 31,133
Exchange rate (in USD per Euro) | $ / €	1.10			1.10
Lexington
Current liabilities
Current operating lease liability	$ 3,383			$ 3,383
Non-current liabilities
Non-current operating lease liability	20,300			20,300
Total lease liabilities	23,683			23,683
Undiscounted Cash Flows
2020 (nine months remaining)	2,535			2,535
2021	3,455			3,455
2022	3,552			3,552
2023	3,650			3,650
2024	4,146			4,146
Thereafter	20,745			20,745
Total minimum lease payments	38,083			38,083
Less: amount of lease payments representing interest	(14,400)			(14,400)
Total lease liabilities	23,683			23,683
Less: current obligations under operating leases	(3,383)			(3,383)
Non-current operating lease liability	$ 20,300			$ 20,300
Amsterdam
Components of lease cost
Lessee, option to extend				true
Number of floor of the facility | floor	7
Number of floors to sub-lease | floor			3			3
Number of floor to take back from the sub-lease | floor		1
Lessor, option to extend				true
Current liabilities
Current operating lease liability	$ 2,319			$ 2,319
Non-current liabilities
Non-current operating lease liability	10,150			10,150
Total lease liabilities	12,469			12,469
Undiscounted Cash Flows
2020 (nine months remaining)	1,855			1,855
2021	1,855			1,855
2022	1,855			1,855
2023	1,855			1,855
2024	1,855			1,855
Thereafter	12,833			12,833
Total minimum lease payments	22,108			22,108
Less: amount of lease payments representing interest	(9,639)			(9,639)
Total lease liabilities	12,469			12,469
Less: current obligations under operating leases	(2,319)			(2,319)
Non-current operating lease liability	10,150			10,150
Other
Current liabilities
Current operating lease liability	198			198
Non-current liabilities
Non-current operating lease liability	68			68
Total lease liabilities	266			266
Undiscounted Cash Flows
2020 (nine months remaining)	139			139
2021	139			139
Total minimum lease payments	278			278
Less: amount of lease payments representing interest	(12)			(12)
Total lease liabilities	266			266
Less: current obligations under operating leases	(198)			(198)
Non-current operating lease liability	$ 68			$ 68