CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net Income	$ 36,593	$ 27,581	$ 28,925
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	8,677	8,953	8,823
Deferred income taxes	(1,374)	(247)	(1,431)
Stock-based compensation	1,602	1,566	1,841
Impairment of investment	0	345	2,413
Net change in accrued interest, premiums, and discounts on investments	(195)	(37)	100
Other	49	0	17
Net Cash Provided By Used In Operating Activities Before Changes In Operating Capital, Total	45,352	38,161	40,688
Changes in operating assets and liabilities:
Accounts receivable	88	(546)	371
Inventories	(339)	756	(1,749)
Prepaid expenses and other current assets	(18)	(247)	1,786
Other non-current assets	75	(673)	(103)
Accounts payable and accrued liabilities	213	(324)	(492)
Accrued income and other taxes	336	81	(128)
Other non-current liabilities	1,330	195	54
Net Cash Provided by Operating Activities, Total	47,037	37,403	40,427
Cash Flows From Investing Activities:
Property, plant and equipment additions	(9,677)	(10,639)	(9,323)
Purchase of investments	(69,193)	(30,799)	(168)
Proceeds from sale of investments	0	210	0
Proceeds from maturities of investments	58,000	5,000	13,400
Net Cash Provided by (Used in) Investing Activities, Total	(20,870)	(36,228)	3,909
Cash Flows From Financing Activities:
Shares tendered for employees' withholding taxes on stock-based compensation	(7,735)	(1,112)	(154)
Tax benefit related to stock-based compensation	0	0	156
Purchase of treasury stock	0	(1,276)	(30,698)
Dividends paid	(8,318)	(7,111)	(6,069)
Net Cash Used in Financing Activities, Total	(16,053)	(9,499)	(36,765)
Net change in cash and cash equivalents	10,114	(8,324)	7,571
Cash and cash equivalents, beginning of year	20,022	28,346	20,775
Cash and cash equivalents, end of year	30,136	20,022	28,346
Cash paid for:
Income taxes, net of refunds	4,959	10,750	12,900
Non-cash financing activities:
Non-cash effect of stock option exercises	$ 10,237	$ 0	$ 0