Income Taxes - Tax Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income (loss) before income taxes
United States	$ 52,172	$ 40,752	$ 8,880
Foreign	2,533	3,079	2,144
Income before income taxes	54,705	43,831	11,024
Current:
Federal	41	430
State	112	32	49
Foreign	323	230	(217)
Total current	476	692	(168)
Deferred:
Federal	8,108	(82,048)
State	425	(1,698)
Foreign	(189)	(74)	191
Total deferred	8,344	(83,820)	191
Income tax provision (benefit)	8,820	(83,128)	23
Reconciliation of income taxes at the United States Federal statutory rate to the effective income tax rate
Income taxes at the United States statutory rate	11,488	15,341	3,859
State income taxes	299	203	32
Unrecognized tax benefits	(345)	(285)	(615)
Effect of change in valuation allowance	(441)	(115,831)	(7,765)
Foreign income tax rate differentials	73	(312)	233
Unremitted earnings of foreign subsidiaries		(8,933)	305
Stock options	(715)	(10,342)	264
Credit expirations			3,565
Repatriation of foreign earnings		4,556
Recognition of equity NOL's		(1,165)
Rate change	160	42,531
Credit generation	(3,530)	(8,778)
Discrete items, net	972	31
Other, net	859	(144)	145
Income tax provision (benefit)	8,820	(83,128)	$ 23
Significant components of long-term deferred income taxes
Federal net operating loss carryforwards	42,397	56,646
State net operating loss carryforwards	1,387	1,586
Foreign net operating loss carryforwards	641	758
Federal tax credit carryforwards	16,200	14,312
State tax credit carryforwards	6,489	6,908
Unremitted earnings of foreign subsidiaries		(21)
Intangible assets	(29)	116
Property, plant and equipment	5,924	5,838
Accrued compensation	97	45
Inventories	3,713	3,798
Stock compensation	2,760	2,351
Warranty	1,090	980
Deferred revenue	1,004	187
Other	(2,899)	(3,220)
Deferred taxes, gross	78,774	90,284
Valuation allowance	(6,835)	(7,136)
Deferred taxes, net	$ 71,939	$ 83,148