Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 424,683,939	$ 437,883,097	$ 406,791,922
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	104,702,974	102,186,814	99,570,908
Gain on disposal of assets	(155,150)	(577,200)	(188,150)
Loss on disposal of assets	588,941	108,927	299,174
Gain on sale of investments	(660,643)	(2,538,729)	(1,309,166)
Loss on sale of investments	176,360	532,494	375,388
Deferred income taxes	(3,358,537)	(4,414,739)	(14,996,179)
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	21,671,192	18,305,981	18,376,965
Change in operating assets and liabilities:
Accounts receivable	(21,872,527)	17,583,989	(19,530,043)
Inventories	(23,660,256)	(8,516,016)	(27,454,146)
Prepaid expenses and other	(3,646,457)	(11,268,677)	16,183,673
Accounts payable	4,743,601	2,911,849	9,934,837
Accrued liabilities	2,753,427	220,856	12,947,597
Net cash flows from operating activities	505,966,864	552,418,646	501,002,780
Activity in available-for-sale securities:
Sales proceeds	57,139,135	55,248,551	30,207,523
Maturities and calls	125,013,589	181,892,136	23,100,000
Purchases	(153,257,603)	(332,106,362)	(29,874,960)
Plant and equipment additions	(84,580,255)	(85,990,570)	(104,040,919)
Proceeds from sale of plant and equipment	2,001,315	738,093	249,757
(Increase) Decrease in other assets	(3,027,263)	(5,603,042)	2,646,029
Net cash used for investing activities	(56,711,082)	(185,821,194)	(77,712,570)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of long-term debt	0	(78,000,000)	(107,625,000)
Issuance of common stock from stock plan transactions	77,821,151	66,837,820	47,770,467
Cash dividends paid	(116,309,197)	(116,566,639)	(108,815,040)
Repurchases of common stock	(331,471,392)	(591,577,851)	(231,363,216)
Net cash used for financing activities	(369,959,438)	(719,306,670)	(400,032,789)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	79,296,344	(352,709,218)	23,257,421
CASH AND CASH EQUIVALENTS, Beginning of year	217,025,278	569,734,496	546,477,075
CASH AND CASH EQUIVALENTS, End of year	$ 296,321,622	$ 217,025,278	$ 569,734,496