Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 406,791,922	$ 347,591,276	$ 318,469,859
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	99,570,908	88,587,430	80,599,167
Gain on disposal of assets	(188,150)	(146,261)	(10,900)
Loss on disposal of assets	299,174	1,080,486	455,950
Gain on sale of investments	(1,309,166)	(4,239,895)	(9,666,482)
Loss on sale of investments	375,388	3,919,698	2,705,601
Deferred income taxes	(14,996,179)	22,498,361	13,058,458
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	18,376,965	19,192,699	21,430,991
Change in operating assets and liabilities:
Accounts receivable	(19,530,043)	(15,622,345)	(27,960,696)
Inventories	(27,454,146)	(14,616,026)	(32,937,525)
Prepaid expenses and other	16,183,673	4,399,366	(9,530,002)
Accounts payable	9,934,837	13,609,856	(5,103,209)
Accrued liabilities	12,947,597	10,793,540	2,904,940
Net cash flows from operating activities	501,002,780	477,048,185	354,416,152
Activity in available-for-sale securities:
Sales proceeds	30,207,523	87,293,155	58,517,164
Maturities and calls	23,100,000	5,500,000	0
Purchases	(29,874,960)	(216,670,674)	(47,513,972)
Plant and equipment additions	(104,040,919)	(120,955,614)	(97,941,762)
Proceeds from sale of plant and equipment	249,757	665,191	43,544
Increase in other assets	2,646,029	(7,278,166)	(2,842,635)
Net cash used for investing activities	(77,712,570)	(251,446,108)	(89,737,661)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of long-term debt	(107,625,000)	(47,500,000)	(32,500,000)
Issuance of common stock from stock plan transactions	47,770,467	81,310,048	30,168,496
Cash dividends paid	(108,815,040)	(101,131,356)	(96,990,439)
Repurchases of common stock	(231,363,216)	(163,361,221)	(111,228,825)
Net cash used for financing activities	(400,032,789)	(230,682,529)	(210,550,768)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	23,257,421	(5,080,452)	54,127,723
CASH AND CASH EQUIVALENTS, Beginning of year	546,477,075	551,557,527	497,429,804
CASH AND CASH EQUIVALENTS, End of year	$ 569,734,496	$ 546,477,075	$ 551,557,527