Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income	$ 42,572	$ 54,498	$ 53,376
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	17,655	15,007	13,035
Amortization of bond premiums	13	47	249
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	174	179	(25)
Provision for excess and obsolete inventories	152	264	625
Share-based compensation	7,374	6,458	4,357
(Gain) loss on disposition of assets	(12)	45	(20)
Foreign currency transaction loss (gain)	55	(59)	(22)
Interest income on note receivable	(27)	(25)	(28)
Deferred income taxes	2,849	(1,554)	825
Changes in assets and liabilities:
Accounts receivable	(2,832)	(7,516)	7,048
Income tax receivable	(4,461)	4,596	(1,537)
Inventories	(5,598)	(23,698)	(9,478)
Prepaid expenses and other	(528)	98	(83)
Accounts payable	(1,176)	3,043	654
Deferred revenue	412	258	417
Accrued liabilities and donations	(1,766)	6,353	(5,470)
Net cash provided by operating activities	54,856	57,994	63,923
Investing Activities
Capital expenditures	(37,268)	(41,713)	(26,604)
Cash paid in business combination	(6,377)	0	0
Proceeds from sale of property, plant and equipment	13	10	28
Investment in certificates of deposits	(7,200)	(5,280)	(4,112)
Maturities of certificates of deposits	10,080	7,912	10,560
Purchase of investments held to maturity	(9,001)	(13,241)	(10,384)
Maturities of investments	14,570	19,700	10,021
Proceeds from called investments	495	1,500	3,514
Principal payments from note receivable	53	60	52
Net cash used in investing activities	(34,635)	(31,052)	(16,925)
Financing Activities
Borrowings under revolving credit facility	0	0	761
Payments under revolving credit facility	0	0	(761)
Stock options exercised	4,987	2,259	2,063
Repurchase of stock	(26,846)	(16,620)	(19,317)
Employee taxes paid by withholding shares	(1,097)	(1,614)	(823)
Cash dividends paid to stockholders	(16,728)	(13,663)	(12,676)
Net cash used in financing activities	(39,684)	(29,638)	(30,753)
Net (decrease) increase in cash and cash equivalents	(19,463)	(2,696)	16,245
Cash and cash equivalents, beginning of year	21,457	24,153	7,908
Cash and cash equivalents, end of year	$ 1,994	$ 21,457	$ 24,153