Income taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Line Items]
Loss before income taxes		$ (25,284)	$ (57,363)
Income Tax Expense (Benefit)		656	$ (1,704)
Operating Loss Carryforwards		$ 46,400
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%	35.00%
Deferred Tax Assets, Valuation Allowance		$ 18,918	$ 11,315
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		$ 18,900
Benefit as a result of enactment of the 2017 Tax Act	$ 1,400
Percentage of tax earnings		80.00%
Annual limitation of NOLs		$ 1,300
No Expiration [Member]
Income Tax [Line Items]
Operating Loss Carryforwards		41,000
Expiring Through 2023 [Member]
Income Tax [Line Items]
Operating Loss Carryforwards		5,400
Operating Loss Limitation		1,300
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Operating Loss Carryforwards		$ 23,700