Statements of Cash Flows - USD ($)	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
OPERATING ACTIVITIES
Net income	$ 12,948,869	$ 12,292,315	$ 14,368,354
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, Depletion and Amortization	1,147,451	850,970	934,371
Stock-based compensation	22,000	21,160	58,960
Excess tax benefits	(32,616)	(352,294)	(24,288)
Deferred income taxes	6,144	295,827	155,713
Changes in operating assets and liabilities
Accounts receivable	(1,192,716)	719,888	(632,400)
Inventories	(153,065)	537,259	(535,159)
Prepaid expenses and other assets	127,241	(159,611)	241,363
Accounts payable and accrued expenses	77,924	(611,290)	303,152
Deferred revenue	(572,072)	714,805
Net cash provided by operating activities	12,379,160	14,309,029	14,870,066
INVESTING ACTIVITIES
Purchases of fixed assets	(519,835)	(287,275)	(185,007)
Purchases of marketable securities	(11,528,240)	(14,568,089)	(8,997,086)
Proceeds from maturities of marketable securities	19,400,000	19,725,000	12,160,000
Net cash provided by investing activities	7,351,925	4,869,636	2,977,907
FINANCING ACTIVITIES
Proceeds from sale of common stock	247,110	292,909	302,701
Excess tax benefits	32,616	352,294	24,288
Repurchase of common stock		(2,311,673)
Payment of dividends to shareholders	(19,346,040)	(19,414,864)	(10,000,000)
Net cash used in financing activities	(19,066,314)	(21,081,334)	(9,673,011)
Increase (decrease) in cash and cash equivalents	664,771	(1,902,669)	8,174,962
Cash and cash equivalents at beginning of year	7,534,593	9,437,262	1,262,300
Cash and cash equivalents at end of year	8,199,364	7,534,593	9,437,262
Supplemental disclosures of cash flow information:
Cash paid during the year for income taxes	$ 6,040,000	$ 5,810,000	$ 6,800,000