CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net income	$ 7,726,000	$ 8,898,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	3,093,000	2,763,000
Deferred income taxes	186,000	(500,000)
Stock-based compensation	497,000	378,000
Net change in unrealized gains and losses on investments	(61,000)	1,287,000
Net change in accrued interest, premiums, and discounts on investments	61,000	77,000
Other	25,000	0
Other operating activities	11,527,000	12,903,000
Changes in operating assets and liabilities:
Accounts receivable	(3,790,000)	(3,931,000)
Inventories	779,000	841,000
Prepaid expenses	1,483,000	1,178,000
Other non-current assets	78,000	58,000
Accounts payable and accrued liabilities	(3,406,000)	(530,000)
Accrued income and other taxes	416,000	2,217,000
Other non-current liabilities	317,000	95,000
Net cash provided operating activities	7,404,000	12,831,000
Cash flows from investing activities:
Property, plant and equipment additions	(3,044,000)	(3,574,000)
Purchase of investments	(7,392,000)	(12,392,000)
Proceeds from sale of investments	65,000	329,000
Proceeds from maturities of investments	13,728,000	13,951,000
Net cash provided investing activities	3,357,000	(1,686,000)
Cash flows from financing activities:
Purchase of treasury stock	0	(9,245,000)
Shares tendered for employees' withholding taxes on stock-based compensation	(585,000)	0
Dividends paid	(3,199,000)	(2,868,000)
Net cash provided financing activities	(3,784,000)	(12,113,000)
Net change in cash and cash equivalents	6,977,000	(968,000)
Cash and cash equivalents at beginning of period	22,450,000	45,048,000
Cash and cash equivalents at end of period	29,427,000	44,080,000
Cash paid for:
Income taxes	79,000	54,000
Non-cash financing activities:
Non-cash effect of stock option exercises	$ 6,012,000	$ 0