Income taxes - Narrative (Details) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Valuation allowance, increase (decrease)	$ 700,000	$ 16,800,000
Impact of adoption of new accounting standard ASU 2016-16				$ (1,824,000)
Unrecognized tax benefits	20,652,000	$ 26,196,000		$ 25,539,000	$ 23,606,000
Unrecognized tax benefits that would impact the effective tax rate	100,000
Unrecognized tax benefits, income tax penalties and interest expense	0
Domestic Tax Authority
Income Tax Examination [Line Items]
Operating loss carryforwards	290,400,000
State and Local Jurisdiction
Income Tax Examination [Line Items]
Operating loss carryforwards	52,100,000
Accounting Standards Update 2016-16
Income Tax Examination [Line Items]
Impact of adoption of new accounting standard ASU 2016-16			$ 1,800,000
Research Tax Credit Carryforward | Domestic Tax Authority
Income Tax Examination [Line Items]
Tax credit carryforward	10,200,000
Research Tax Credit Carryforward | State and Local Jurisdiction
Income Tax Examination [Line Items]
Tax credit carryforward	18,800,000
Research Tax Credit Carryforward | Foreign Tax Authority
Income Tax Examination [Line Items]
Tax credit carryforward	$ 10,500,000