Taxes - Constituents of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Non-current deferred tax assets:
Stock-based compensation expense	$ 13,818	$ 11,664
Federal and state tax credit carryforwards	7,395	6,707
Bonuses, commissions, and other compensation	5,470	5,704
Inventory and revenue related	3,233	3,415
Depreciation	2,475	2,279
Other	2,425	3,012
Gross non-current deferred tax assets	34,816	32,781
Non-current deferred tax liabilities:
Nondeductible intangible assets	(44)	(87)
Gross non-current deferred tax liabilities	(44)	(87)
Valuation allowance	(6,112)	(5,309)
Net non-current deferred tax assets	28,660	27,385
Non-current deferred tax liabilities:
Other	(962)	(312)
Net non-current deferred tax liabilities	$ 962	$ 312