Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 318,469,859	$ 288,604,579	$ 222,929,949
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	80,599,167	77,376,305	62,854,155
Gain on disposal of assets	(10,900)	(44,410)	(54,520)
Loss on disposal of assets	455,950	640,918	2,162,610
Gain on sale of investments	(9,666,482)	(16,742,536)	(19,012,429)
Loss on sale of investments	2,705,601	1,149,627	882,918
Deferred income taxes	13,058,458	14,817,959	(7,726,800)
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	21,430,991	21,276,991	17,427,233
Excess tax benefits from stock based compensation	(2,837,727)	(5,502,339)	(4,445,064)
Change in operating assets and liabilities:
Accounts receivable	(27,960,696)	(24,962,114)	(22,347,450)
Inventories	(32,937,525)	(21,683,721)	39,856,102
Prepaid expenses and other	(9,530,002)	(21,946,622)	(2,374,913)
Accounts payable	(5,103,209)	14,946,662	13,310,319
Accrued liabilities	2,904,940	(708,190)	13,876,720
Net cash provided by operating activities	351,578,425	327,223,109	317,338,830
Activity in available-for-sale securities:
Sales proceeds	58,517,164	75,613,807	82,862,644
Maturities and calls	0	0	98,142,408
Purchases	(47,513,972)	(80,887,357)	(60,543,153)
Plant and equipment additions	(97,941,762)	(72,518,987)	(55,380,457)
Proceeds from sale of plant and equipment	43,544	223,833	386,399
Acquisition of businesses, net of cash acquired	0	0	(698,150,900)
Increase in other assets	(2,842,635)	(2,144,779)	(573,560)
Net cash (used for) investing activities	(89,737,661)	(79,713,483)	(633,256,619)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from borrowings on Credit Agreement	0	0	275,000,000
Repayment of long-term debt	(32,500,000)	(7,500,000)	(1,875,000)
Deferred financing costs	0	0	(956,441)
Issuance of common stock from stock plan transactions	30,168,496	59,969,006	38,377,571
Cash dividends paid	(96,990,439)	(87,632,009)	(79,160,345)
Repurchases of common stock	(111,228,825)	(30,010,882)	0
Excess tax benefits from stock based compensation	2,837,727	5,502,339	4,445,064
Net cash provided by (used for) financing activities	(207,713,041)	(59,671,546)	235,830,849
NET INCREASE IN CASH AND CASH EQUIVALENTS	54,127,723	187,838,080	(80,086,940)
CASH AND CASH EQUIVALENTS, Beginning of year	497,429,804	309,591,724	389,678,664
CASH AND CASH EQUIVALENTS, End of year	$ 551,557,527	$ 497,429,804	$ 309,591,724