CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows From Operating Activities:
Net income	$ 32,115,000	$ 36,761,000	$ 34,255,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11,652,000	10,853,000	9,123,000
Deferred income taxes	2,282,000	1,809,000	(625,000)
Stock-based compensation	1,731,000	1,682,000	1,659,000
Net change in unrealized gains and losses on investments	(1,093,000)	(135,000)	1,399,000
Net change in accrued interest, premiums, and discounts on investments	112,000	(281,000)	47,000
Other	21,000	(6,000)	(18,000)
Adjustments to reconcile net income to net cash provided by operating activities	46,820,000	50,683,000	45,840,000
Changes in operating assets and liabilities:
Accounts receivable	2,438,000	(1,872,000)	62,000
Inventories	(8,205,000)	(8,521,000)	(4,218,000)
Prepaid expenses and other current assets	(1,323,000)	697,000	(43,000)
Other non-current assets	(275,000)	(425,000)	(87,000)
Accounts payable and accrued liabilities	2,095,000	1,254,000	725,000
Accrued income and other taxes	17,000	(200,000)	(127,000)
Other non-current liabilities	(2,347,000)	849,000	1,084,000
Net cash provided by used in operating activities	38,970,000	42,465,000	43,236,000
Cash Flows From Investing Activities:
Property, plant and equipment additions	(21,886,000)	(20,446,000)	(17,507,000)
Purchase of investments	(45,768,000)	(83,721,000)	(28,472,000)
Proceeds from sale of investments	899,000	0	0
Proceeds from maturities of investments	35,923,000	59,331,000	40,898,000
Net cash provided by used in investing activities	(30,832,000)	(44,836,000)	(5,081,000)
Cash Flows From Financing Activities:
Shares tendered for employees' withholding taxes on stock-based compensation	(55,000)	(579,000)	(90,000)
Purchase of treasury stock	(18,831,000)	0	0
Dividends paid	(12,100,000)	(10,755,000)	(9,448,000)
Net cash provided by used in financing activities	(30,986,000)	(11,334,000)	(9,538,000)
Net change in cash and cash equivalents	(22,598,000)	(13,705,000)	28,617,000
Cash and cash equivalents, beginning of year	45,048,000	58,753,000	30,136,000
Cash and cash equivalents, end of year	22,450,000	45,048,000	58,753,000
Cash paid for:
Income taxes, net of refunds	$ 5,565,000	$ 4,178,000	$ 9,858,000