Statements of Cash Flows (USD $)	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
OPERATING ACTIVITIES
Net income	$ 11,135,875	$ 11,828,838	$ 11,381,095
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	844,339	647,163	500,121
Stock-based compensation	53,200	66,720	80,160
Excess tax benefits	(57,472)	(15,734)	0
Deferred income taxes	121,881	51,262	(12,850)
Changes in operating assets and liabilities:
Accounts receivable	189,821	163,445	911,399
Inventories	129,259	(107,216)	114,481
Prepaid expenses and other assets	141,871	201,705	25,454
Accounts payable and accrued expenses	(157,350)	(190,881)	(187,950)
Net cash provided by operating activities	12,401,424	12,645,302	12,811,910
INVESTING ACTIVITIES
Purchases of fixed assets	(160,718)	(1,824,324)	(1,480,237)
Purchases of marketable securities	(22,753,916)	(27,209,753)	(18,501,362)
Proceeds from maturities and sales of marketable securities	10,055,000	17,194,000	7,761,980
Net cash used in investing activities	(12,859,634)	(11,840,077)	(12,219,619)
FINANCING ACTIVITIES
Net proceeds from sale of common stock	416,760	144,188	36
Excess tax benefits	57,472	15,734	0
Repurchase of common stock	(1,263,405)	0	0
Net cash provided by financing activities	(789,173)	159,922	36
Increase (decrease) in cash and cash equivalents	(1,247,383)	965,147	592,327
Cash and cash equivalents at beginning of period	2,509,683	1,544,536	952,209
Cash and cash equivalents at end of period	1,262,300	2,509,683	1,544,536
Supplemental disclosures of cash flow information:
Cash (refunded) paid during the period for income taxes	$ 5,263,033	$ 5,202,616	$ 5,207,565