Income taxes - Components of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets:
Net operating loss carryforwards	$ 56,828	$ 44,912
Federal and state credits	28,328	26,170
Reserves, accruals and other	11,265	9,698
Fixed assets and intangibles	2,398	1,259
Total deferred tax assets	98,819	82,039
Valuation allowance	(92,891)	(76,101)
Total deferred tax assets, net of valuation allowance	5,928	5,938
Less deferred tax liabilities:
Acquired intangibles	(313)	(2,054)
Property, plant and equipment	(4,754)	(3,338)
Net deferred tax assets	861	546
Reported as:
Long term deferred tax assets, included within other long-term assets	861	652
Long term deferred income tax liabilities, included within noncurrent liabilities	0	(106)
Net deferred tax assets	$ 861	$ 546