Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities
Net income	$ 53,376	$ 45,728	$ 44,158
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	13,035	11,741	11,553
Amortization of bond premiums	249	266	688
Allowance for Doubtful Accounts Receivable, Period Increase (Decrease)	(25)	(48)	(22)
Provision for excess and obsolete inventories	625	178	135
Share-based compensation	4,357	2,891	2,178
Gain on disposition of assets	(20)	(59)	(305)
Foreign currency transaction (gain) loss	(22)	139	74
Interest income on note receivable	(28)	(30)	(36)
Deferred income taxes	825	1,172	(2,111)
Write-off of note receivable	0	0	0
Changes in assets and liabilities:
Accounts receivable	7,048	(5,884)	(5,007)
Income tax receivable	(1,537)	312	(257)
Inventories	(9,478)	(1,059)	(5,613)
Prepaid expenses and other	(83)	76	(305)
Accounts payable	654	(5,109)	3,512
Deferred revenue	417	189	782
Accrued liabilities and donations	(5,470)	4,852	4,094
Net cash provided by operating activities	63,923	55,355	53,518
Investing Activities
Capital expenditures	(26,604)	(20,967)	(16,127)
Proceeds from sale of property, plant and equipment	28	63	319
Investment in certificates of deposits	(4,112)	(6,680)	(9,940)
Maturities of certificates of deposits	10,560	6,098	9,310
Purchase of investments held to maturity	(10,384)	(14,183)	(6,880)
Maturities of investments	10,021	11,408	14,197
Proceeds from called investments	3,514	1,013	3,029
Principal payments from note receivable	52	54	63
Net cash used in investing activities	(16,925)	(23,194)	(6,029)
Financing Activities
Borrowings under revolving credit facility	761	0	0
Payments under revolving credit facility	(761)	0	0
Stock options exercised	2,063	2,795	1,318
Repurchase of stock	(19,317)	(36,558)	(29,066)
Employee taxes paid by withholding shares	(823)	(585)	(218)
Cash dividends paid to stockholders	(12,676)	(11,857)	(9,656)
Net cash used in financing activities	(30,753)	(46,205)	(37,622)
Net increase (decrease) in cash and cash equivalents	16,245	(14,044)	9,867
Cash and cash equivalents, beginning of year	7,908	21,952	12,085
Cash and cash equivalents, end of year	$ 24,153	$ 7,908	$ 21,952