CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash Flows From Operating Activities:
Net income	$ 33,055,000	$ 32,115,000	$ 36,761,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	12,885,000	11,652,000	10,853,000
Deferred income taxes	(3,686,000)	2,282,000	1,809,000
Stock-based compensation	2,312,000	1,731,000	1,682,000
Net change in unrealized gains and losses on investments	(1,472,000)	(1,093,000)	(135,000)
Net change in accrued interest, premiums, and discounts on investments	632,000	112,000	(281,000)
Other	22,000	21,000	(6,000)
Adjustments to reconcile net income to net cash provided by operating activities	43,748,000	46,820,000	50,683,000
Changes in operating assets and liabilities:
Accounts receivable	(4,579,000)	2,438,000	(1,872,000)
Inventories	(480,000)	(8,205,000)	(8,521,000)
Prepaid expenses and other current assets	423,000	(1,323,000)	697,000
Other non-current assets	11,000	(275,000)	(425,000)
Accounts payable and accrued liabilities	(656,000)	2,095,000	1,254,000
Accrued income and other taxes	(165,000)	17,000	(200,000)
Other non-current liabilities	496,000	(2,347,000)	849,000
Net cash provided by used in operating activities	38,798,000	39,220,000	42,465,000
Cash Flows From Investing Activities:
Property, plant and equipment additions	(15,828,000)	(21,886,000)	(20,446,000)
Purchase of investments	(23,158,000)	(45,768,000)	(83,721,000)
Proceeds from sale of investments	793,000	899,000	0
Proceeds from maturities of investments	40,189,000	35,923,000	59,331,000
Net cash provided by used in investing activities	1,996,000	(30,832,000)	(44,836,000)
Cash Flows From Financing Activities:
Shares tendered for employees' withholding taxes on stock-based compensation	(585,000)	(55,000)	(579,000)
Purchase of treasury stock	(16,988,000)	(18,831,000)	0
Dividends paid	(13,407,000)	(12,100,000)	(10,755,000)
Net cash provided by used in financing activities	(30,980,000)	(30,986,000)	(11,334,000)
Net change in cash and cash equivalents	9,814,000	(22,598,000)	(13,705,000)
Cash and cash equivalents, end of year	32,264,000	22,450,000	45,048,000
Cash and cash equivalents, beginning of year	22,450,000	45,048,000	58,753,000
Cash paid for:
Income taxes, net of refunds	7,744,000	$ 5,565,000	$ 4,178,000
Non-cash financing activities:
Non-cash effect of stock option exercises	$ 6,012,000