Income Taxes - Tax Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income (loss) before income taxes
United States	$ 40,752	$ 8,880	$ 12,708
Foreign	3,079	2,144	2,497
Income before income taxes	43,831	11,024	15,205
Current:
Federal	430
State	32	49	(33)
Foreign	230	(217)	374
Total current	692	(168)	341
Deferred:
Federal	(82,048)
State	(1,698)
Foreign	(74)	191	186
Total deferred	(83,820)	191	186
Income tax provision	(83,128)	23	527
Reconciliations of income taxes at the United States Federal statutory rate to the effective income tax rate
Income tax benefit at the United States statutory rate	15,341	3,859	5,322
State income taxes	203	32	(22)
Unrecognized tax benefits	(285)	(615)	(174)
Effect of change in valuation allowance	(115,831)	(7,765)	(5,676)
Foreign income tax rate differentials	(312)	233	600
Unremitted earnings of foreign subsidiaries	(8,933)	305	(102)
Stock options	(10,342)	264	379
Credit expirations		3,565
Repatriation of foreign earnings	4,556
Recognition of equity NOL's	(1,165)
Rate change	42,531
Credit generation	(8,778)
Discrete items, net	31		(540)
Other, net	(144)	145	740
Income tax provision	(83,128)	23	$ 527
Significant components of long-term deferred income taxes
Federal net operating loss carryforwards	56,646	98,007
State net operating loss carryforwards	1,586	1,546
Foreign net operating loss carryforwards	758	673
Federal tax credit carryforwards	14,312	12,778
State tax credit carryforwards	6,908	2,566
Unremitted earnings of foreign subsidiaries	(21)	(8,913)
Intangible assets	116	229
Property, plant and equipment	5,838	9,774
Accrued compensation	45	97
Inventories	3,798	5,230
Stock compensation	2,351	6,687
Warranty	980	930
Deferred revenue	187
Other	(3,220)	(5,578)
Deferred taxes, gross	90,284	124,026
Valuation allowance	(7,136)	(122,966)
Deferred taxes, net	$ 83,148	$ 1,060