Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 68,566,071	$ 45,430,567
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	19,825,402	13,805,949
Gain on disposal of assets	(54,520)	0
Loss on disposal of assets	218,409	1,495,536
Gain on sale of investments	(5,049,229)	0
Loss on sale of investments	25,808	(2,822,207)
Deferred income taxes	(1,614,318)	(2,180,428)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	4,742,080	3,850,063
Excess tax benefits from stock-based compensation	(702,580)	(341,225)
Change in operating assets and liabilities:
Accounts receivable, net	(28,290,721)	(12,347,433)
Inventories	(8,275,303)	29,860,576
Prepaid expenses and other	534,133	133,880
Accounts payable	7,889,358	(10,198,080)
Accrued liabilities, excluding dividends declared	39,907,356	30,735,336
Net cash provided by operating activities	97,721,946	97,422,534
Activity in available-for-sale securities:
Sales proceeds	23,856,516	10,627,243
Maturities and calls	0	3,000,000
Purchases	(24,487,944)	(33,136,682)
Plant and equipment additions	(16,417,890)	(12,674,970)
Proceeds from sale of plant and equipment	35,005	9,500
(Increase) decrease in other assets	(285,354)	(706,899)
Net cash used for investing activities	(17,299,667)	(32,881,808)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of long-term debt	(1,875,000)	0
Issuance of common stock from stock plan transactions	6,227,631	6,592,272
Cash dividends paid	(20,380,923)	(18,599,926)
Excess tax benefits from stock-based compensation	702,580	341,225
Net cash used for financing activities	(15,325,712)	(11,666,429)
NET INCREASE IN CASH AND CASH EQUIVALENTS	65,096,567	52,874,297
CASH AND CASH EQUIVALENTS, beginning of period	309,591,724	389,678,664
CASH AND CASH EQUIVALENTS, end of period	$ 374,688,291	$ 442,552,961