Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 347,563,621	$ 424,683,939	$ 437,883,097
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	104,739,900	104,702,974	102,186,814
Gain on disposal of assets	(311,510)	(155,150)	(577,200)
Loss on disposal of assets	162,553	588,941	108,927
Gain on sale of investments	(3,163,164)	(660,643)	(2,538,729)
Loss on sale of investments	1,064,508	176,360	532,494
Deferred income taxes	(15,419,722)	(3,358,537)	(4,414,739)
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	30,797,327	21,671,192	18,305,981
Change in operating assets and liabilities:
Accounts receivable	(49,290,457)	(21,872,527)	17,583,989
Inventories	22,725,798	(23,660,256)	(8,516,016)
Prepaid expenses and other	10,493,993	(3,646,457)	(11,268,677)
Accounts payable	(12,854,038)	4,743,601	2,911,849
Accrued liabilities	27,982,962	2,753,427	220,856
Net cash flows from operating activities	464,491,771	505,966,864	552,418,646
Activity in available-for-sale securities:
Sales proceeds	24,455,695	57,139,135	55,248,551
Maturities and calls	142,547,368	125,013,589	181,892,136
Purchases	(73,719,189)	(153,257,603)	(332,106,362)
Plant and equipment additions	(51,706,541)	(84,580,255)	(85,990,570)
Proceeds from sale of plant and equipment	383,429	2,001,315	738,093
Acquisition of businesses, net of cash acquired	(11,216,927)	0	0
Increase in other assets	(4,327,398)	(3,027,263)	(5,603,042)
Net cash from (used for) investing activities	26,416,437	(56,711,082)	(185,821,194)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Proceeds from borrowings on Credit Agreement	75,000,000	0	0
Repayment of borrowings on Credit Agreement	(75,000,000)	0	(78,000,000)
Issuance of common stock from stock plan transactions	41,803,640	77,821,151	66,837,820
Cash dividends paid	(117,181,928)	(116,309,197)	(116,566,639)
Repurchases of common stock	(288,480,506)	(331,471,392)	(591,577,851)
Net cash used for financing activities	(363,858,794)	(369,959,438)	(719,306,670)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	127,049,414	79,296,344	(352,709,218)
CASH AND CASH EQUIVALENTS, Beginning of year	296,321,622	217,025,278	569,734,496
CASH AND CASH EQUIVALENTS, End of year	$ 423,371,036	$ 296,321,622	$ 217,025,278