Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 347,591,276	$ 318,469,859	$ 288,604,579
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	88,587,430	80,599,167	77,376,305
Gain on disposal of assets	(146,261)	(10,900)	(44,410)
Loss on disposal of assets	1,080,486	455,950	640,918
Gain on sale of investments	(4,239,895)	(9,666,482)	(16,742,536)
Loss on sale of investments	3,919,698	2,705,601	1,149,627
Deferred income taxes	22,498,361	13,058,458	14,817,959
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	19,192,699	21,430,991	21,276,991
Excess tax benefits from stock based compensation	(5,583,391)	(2,837,727)	(5,502,339)
Change in operating assets and liabilities:
Accounts receivable	(15,622,345)	(27,960,696)	(24,962,114)
Inventories	(14,616,026)	(32,937,525)	(21,683,721)
Prepaid expenses and other	4,399,366	(9,530,002)	(21,946,622)
Accounts payable	13,609,856	(5,103,209)	14,946,662
Accrued liabilities	10,793,540	2,904,940	(708,190)
Net cash flows from operating activities	471,464,794	351,578,425	327,223,109
Activity in available-for-sale securities:
Sales proceeds	87,293,155	58,517,164	75,613,807
Maturities and calls	5,500,000	0	0
Purchases	(216,670,674)	(47,513,972)	(80,887,357)
Plant and equipment additions	(120,955,614)	(97,941,762)	(72,518,987)
Proceeds from sale of plant and equipment	665,191	43,544	223,833
Increase in other assets	(7,278,166)	(2,842,635)	(2,144,779)
Net cash (used for) investing activities	(251,446,108)	(89,737,661)	(79,713,483)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of long-term debt	(47,500,000)	(32,500,000)	(7,500,000)
Issuance of common stock from stock plan transactions	81,310,048	30,168,496	59,969,006
Cash dividends paid	(101,131,356)	(96,990,439)	(87,632,009)
Repurchases of common stock	(163,361,221)	(111,228,825)	(30,010,882)
Excess tax benefits from stock based compensation	5,583,391	2,837,727	5,502,339
Net cash (used for) financing activities	(225,099,138)	(207,713,041)	(59,671,546)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(5,080,452)	54,127,723	187,838,080
CASH AND CASH EQUIVALENTS, Beginning of year	551,557,527	497,429,804	309,591,724
CASH AND CASH EQUIVALENTS, End of year	$ 546,477,075	$ 551,557,527	$ 497,429,804