Income taxes - Narrative (Details) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Change in prior year deferred balances	$ (1,653,000)	$ 28,262,000	$ (920,000)
Tax Cuts and Jobs Act of 2017 tax expense		30,000,000
Valuation allowance, increase (decrease)	16,800,000	(14,000,000)
Income Tax Examination [Line Items]
Cumulative effect of new accounting standard				$ (1,824,000)
Effective income tax rate reconciliation, share-based compensation, excess tax benefit, amount	8,600,000
Unrecognized tax benefits	26,196,000	$ 25,539,000	$ 23,606,000		$ 20,686,000
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	276,600,000
State and Local Jurisdiction
Income Tax Examination [Line Items]
Operating loss carryforwards	52,000,000
Accounting Standards Update 2016-16
Income Tax Examination [Line Items]
Cumulative effect of new accounting standard				$ 1,800,000
Research Tax Credit Carryforward | Domestic Tax Authority
Income Tax Examination [Line Items]
Tax credit carryforward	9,400,000
Research Tax Credit Carryforward | State and Local Jurisdiction
Income Tax Examination [Line Items]
Tax credit carryforward	17,300,000
Research Tax Credit Carryforward | Foreign Tax Authority
Income Tax Examination [Line Items]
Tax credit carryforward	$ 10,500,000