Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Allowance for uncollectible accounts receivable	$ 467	$ 645
Accrued compensation expense	5,154	6,203
Inventory differences	2,505	1,065
Net operating loss carryforwards	15,741	27,742
Deferred revenue	58	73
Stock-based compensation expense	2,281	2,738
Federal research and development credit carryforward	0	3,524
Foreign tax credits	0	364
Other	8,986	6,984
Total deferred income tax assets	35,192	49,338
Deferred income tax liabilities:
Prepaid expenses	(930)	(782)
Property and equipment	(20,352)	(25,108)
Intangible assets	(28,588)	(35,773)
Other	(1,830)	(1,480)
Total deferred income tax liabilities	(51,700)	(63,143)
Valuation allowance	(4,422)	(3,786)
Net deferred income tax liabilities	(20,930)	(17,591)
Reported as:
Deferred income tax assets - Current	0	8,219
Deferred income tax assets - Long-term	2,359	171
Deferred income tax liabilities - Long-term	(23,289)	(25,981)
Net deferred income tax liabilities	$ (20,930)	$ (17,591)