Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 288,604,579	$ 222,929,949	$ 168,586,840
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	77,376,305	62,854,155	50,179,823
Gain on disposal of assets	(44,410)	(54,520)	(419,382)
Loss on disposal of assets	640,918	2,162,610	952,190
Gain on sale of investments	(16,742,536)	(19,012,429)	(9,584,288)
Loss on sale of investments	1,149,627	882,918	1,216,027
Deferred income taxes	14,817,959	(7,726,800)	843,484
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	21,276,991	17,427,233	16,433,837
Excess tax benefits from stock based compensation	(5,502,339)	(4,445,064)	(815,651)
Change in operating assets and liabilities:
Accounts receivable	(24,962,114)	(22,347,450)	810,022
Inventories	(21,683,721)	39,856,102	28,823,046
Prepaid expenses and other	(21,946,622)	(2,374,913)	14,868,742
Accounts payable	14,946,662	13,310,319	(22,270,571)
Accrued liabilities	(708,190)	13,876,720	8,221,935
Net cash provided by operating activities	327,223,109	317,338,830	257,846,054
Activity in available-for-sale securities:
Sales proceeds	75,613,807	82,862,644	38,831,234
Maturities and calls	0	98,142,408	75,652,000
Purchases	(80,887,357)	(60,543,153)	(112,545,415)
Plant and equipment additions	(72,518,987)	(55,380,457)	(117,474,449)
Proceeds from sale of plant and equipment	223,833	386,399	1,064,614
Acquisition of businesses, net of cash acquired	0	(698,150,900)	0
Increase in other assets	(2,144,779)	(573,560)	(17,602,228)
Net cash (used for) investing activities	(79,713,483)	(633,256,619)	(132,074,244)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from borrowings on Credit Agreement	0	275,000,000	0
Repayment of long-term debt	(7,500,000)	(1,875,000)	0
Deferred financing costs	0	(956,441)	0
Issuance of common stock from stock plan transactions	59,969,006	38,377,571	12,208,881
Cash dividends paid	(87,632,009)	(79,160,345)	(73,387,727)
Repurchases of common stock	(30,010,882)	0	(33,716,725)
Excess tax benefits from stock based compensation	5,502,339	4,445,064	815,651
Net cash provided by (used for) financing activities	(59,671,546)	235,830,849	(94,079,920)
NET INCREASE IN CASH AND CASH EQUIVALENTS	187,838,080	(80,086,940)	31,691,890
CASH AND CASH EQUIVALENTS, Beginning of year	309,591,724	389,678,664	357,986,774
CASH AND CASH EQUIVALENTS, End of year	$ 497,429,804	$ 309,591,724	$ 389,678,664