Statements of Cash Flows - USD ($)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
OPERATING ACTIVITIES
Net income	$ 13,912,672	$ 12,948,869	$ 12,292,315
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	979,652	1,147,451	850,970
Stock-based compensation	40,920	22,000	21,160
Excess tax benefits		(32,616)	(352,294)
Deferred income taxes	96,275	6,144	295,827
Changes in operating assets and liabilities
Accounts receivable	548,023	(1,192,716)	719,888
Inventories	(292,141)	(153,065)	537,259
Prepaid expenses and other assets	(27,877)	127,241	(159,611)
Accounts payable and accrued expenses	37,137	77,924	(611,290)
Deferred revenue	(142,733)	(572,072)	714,805
Net cash provided by operating activities	15,151,928	12,379,160	14,309,029
INVESTING ACTIVITIES
Purchases of fixed assets	(604,800)	(519,835)	(287,275)
Purchases of marketable securities	(18,217,410)	(11,528,240)	(14,568,089)
Proceeds from maturities of marketable securities	19,540,000	19,400,000	19,725,000
Net cash provided by investing activities	717,790	7,351,925	4,869,636
FINANCING ACTIVITIES
Proceeds from sale of common stock	51,040	247,110	292,909
Excess tax benefits		32,616	352,294
Repurchase of common stock			(2,311,673)
Payment of dividends to shareholders	(19,365,040)	(19,346,040)	(19,414,864)
Net cash used in financing activities	(19,314,000)	(19,066,314)	(21,081,334)
(Decrease) increase in cash and cash equivalents	(3,444,282)	664,771	(1,902,669)
Cash and cash equivalents at beginning of year	8,199,364	7,534,593	9,437,262
Cash and cash equivalents at end of year	4,755,082	8,199,364	7,534,593
Supplemental disclosures of cash flow information:
Cash paid during the year for income taxes	$ 5,966,425	$ 6,040,000	$ 5,810,000