Right-of-use asset and lease liabilities - Operating lease liabilities (Details) $ in Thousands	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2020 floor	Feb. 29, 2020 floor	Dec. 31, 2017 floor	Jun. 30, 2020 USD ($) $ / €	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) $ / €	Jun. 30, 2019 USD ($)	Dec. 31, 2017 floor	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Components of lease cost
Operating lease cost				$ 1,289	$ 1,022	$ 2,579	$ 1,919
Variable lease cost				153	101	298	182
Sublease income				(209)	(264)	(457)	(532)
Total lease cost				1,233	859	2,420	1,569
Assets
Operating lease right-of-use assets				26,139	8,600	26,139	8,600		$ 26,797	$ 19,000
Current liabilities
Current operating lease liability				5,495		5,495			5,865
Non-current liabilities
Non-current operating lease liability				30,279		30,279			31,133
Total lease liabilities				$ 35,774		$ 35,774			$ 36,998
Weighted-average remaining lease term, Operating leases				9 years 9 months 18 days		9 years 9 months 18 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				$ 1,731	$ 603	$ 3,155	$ 2,016
Undiscounted Cash Flows
2020 (six months remaining)				2,705		2,705
2021				5,490		5,490
2022				5,446		5,446
2023				5,544		5,544
2024				6,040		6,040
Thereafter				33,845		33,845
Total minimum lease payments				59,070		59,070
Less: amount of lease payments representing interest				(23,296)		(23,296)
Total lease liabilities				35,774		35,774			$ 36,998
Less: current obligations under operating leases				(5,495)		(5,495)			(5,865)
Non-current operating lease liability				$ 30,279		$ 30,279			$ 31,133
Exchange rate (in USD per Euro) | $ / €				1.12		1.12
Lexington
Current liabilities
Current operating lease liability				$ 3,407		$ 3,407
Non-current liabilities
Non-current operating lease liability				20,048		20,048
Total lease liabilities				23,455		23,455
Undiscounted Cash Flows
2020 (six months remaining)				1,697		1,697
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				37,245		37,245
Less: amount of lease payments representing interest				(13,790)		(13,790)
Total lease liabilities				23,455		23,455
Less: current obligations under operating leases				(3,407)		(3,407)
Non-current operating lease liability				20,048		$ 20,048
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				1,894		$ 1,894
Non-current liabilities
Non-current operating lease liability				10,231		10,231
Total lease liabilities				12,125		12,125
Undiscounted Cash Flows
2020 (six months remaining)				947		947
2021				1,894		1,894
2022				1,894		1,894
2023				1,894		1,894
2024				1,894		1,894
Thereafter				13,100		13,100
Total minimum lease payments				21,623		21,623
Less: amount of lease payments representing interest				(9,498)		(9,498)
Total lease liabilities				12,125		12,125
Less: current obligations under operating leases				(1,894)		(1,894)
Non-current operating lease liability				10,231		10,231
Other
Current liabilities
Current operating lease liability				194		194
Non-current liabilities
Total lease liabilities				194		194
Undiscounted Cash Flows
2020 (six months remaining)				61		61
2021				141		141
Total minimum lease payments				202		202
Less: amount of lease payments representing interest				(8)		(8)
Total lease liabilities				194		194
Less: current obligations under operating leases				$ (194)		$ (194)