Statements of Cash Flows (USD $)	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
OPERATING ACTIVITIES
Net income	$ 11,828,838	$ 11,381,095	$ 13,360,945
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	647,163	500,121	411,547
Stock-based compensation	66,720	80,160	76,720
Excess tax benefits	(15,734)	0	(230,173)
Deferred income taxes	51,262	(12,850)	294,384
Changes in operating assets and liabilities:
Accounts receivable	163,445	911,399	625,325
Inventories	(107,216)	114,481	(1,637,430)
Prepaid expenses and other assets	201,705	25,454	(404,012)
Accounts payable and accrued expenses	(190,881)	(187,950)	332,334
Deferred revenue	0	0	(20,833)
Net cash provided by operating activities	12,645,302	12,811,910	12,808,807
INVESTING ACTIVITIES
Purchases of fixed assets	(1,824,324)	(1,480,237)	(732,800)
Purchases of marketable securities	(27,209,753)	(18,501,362)	(14,742,032)
Proceeds from maturities and sales of marketable securities	17,194,000	7,761,980	1,580,068
Net cash used in investing activities	(11,840,077)	(12,219,619)	(13,894,764)
FINANCING ACTIVITIES
Net proceeds from sale of common stock	144,188	36	418,705
Excess tax benefits	15,734	0	230,173
Net cash provided by financing activities	159,922	36	648,878
Increase (decrease) in cash and cash equivalents	965,147	592,327	(437,079)
Cash and cash equivalents at beginning of period	1,544,536	952,209	1,389,288
Cash and cash equivalents at end of period	2,509,683	1,544,536	952,209
Supplemental disclosures of cash flow information:
Cash (refunded) paid during the period for income taxes	$ 5,202,616	$ 5,207,565	$ 6,303,598