Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Net long-term deferred assets and (liabilities) relating to:
Accounts receivable and inventory reserves	$ 401	$ 318
Warranty accrual	3,105	2,698
Other accruals	2,445	1,395
Share-based compensation	1,697	1,432
Donations	80	152
Other, net	851	698
Total deferred income tax assets	8,579	6,693
Property & equipment	(19,405)	(14,670)
Total deferred income tax liabilities	(19,405)	(14,670)
Net deferred income tax liabilities	$ 10,826	$ 7,977