Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 437,883,097	$ 406,791,922	$ 347,591,276
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	102,186,814	99,570,908	88,587,430
Gain on disposal of assets	(577,200)	(188,150)	(146,261)
Loss on disposal of assets	108,927	299,174	1,080,486
Gain on sale of investments	(2,538,729)	(1,309,166)	(4,239,895)
Loss on sale of investments	532,494	375,388	3,919,698
Deferred income taxes	(4,414,739)	(14,996,179)	22,498,361
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	18,305,981	18,376,965	19,192,699
Change in operating assets and liabilities:
Accounts receivable	17,583,989	(19,530,043)	(15,622,345)
Inventories	(8,516,016)	(27,454,146)	(14,616,026)
Prepaid expenses and other	(11,268,677)	16,183,673	4,399,366
Accounts payable	2,911,849	9,934,837	13,609,856
Accrued liabilities	220,856	12,947,597	10,793,540
Net cash flows from operating activities	552,418,646	501,002,780	477,048,185
Activity in available-for-sale securities:
Sales proceeds	55,248,551	30,207,523	87,293,155
Maturities and calls	181,892,136	23,100,000	5,500,000
Purchases	(332,106,362)	(29,874,960)	(216,670,674)
Plant and equipment additions	(85,990,570)	(104,040,919)	(120,955,614)
Proceeds from sale of plant and equipment	738,093	249,757	665,191
(Increase) Decrease in other assets	(5,603,042)	2,646,029	(7,278,166)
Net cash used for investing activities	(185,821,194)	(77,712,570)	(251,446,108)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of long-term debt	(78,000,000)	(107,625,000)	(47,500,000)
Issuance of common stock from stock plan transactions	66,837,820	47,770,467	81,310,048
Cash dividends paid	(116,566,639)	(108,815,040)	(101,131,356)
Repurchases of common stock	(591,577,851)	(231,363,216)	(163,361,221)
Net cash used for financing activities	(719,306,670)	(400,032,789)	(230,682,529)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(352,709,218)	23,257,421	(5,080,452)
CASH AND CASH EQUIVALENTS, Beginning of year	569,734,496	546,477,075	551,557,527
CASH AND CASH EQUIVALENTS, End of year	$ 217,025,278	$ 569,734,496	$ 546,477,075