Condensed Consolidated Statements of Cash Flows (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 97,527,964	$ 87,111,123
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	27,566,632	25,138,931
(Gain) loss on disposal of assets	2,010,463	397,298
Gain on sale of investments	(8,567,087)	(4,806,646)
Loss on sale of investments	714,756	570,746
Deferred income taxes	(3,024,452)	1,729,418
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	8,077,955	7,991,705
Excess tax benefits from stock-based compensation	(1,495,850)	(586,535)
Change in operating assets and liabilities:
Accounts receivable, net	(10,607,330)	(19,558,762)
Inventories	43,928,296	(3,576,693)
Prepaid expenses and other	329,541	3,305,799
Accounts payable	589,628	(11,955,646)
Accrued liabilities, excluding dividends declared	15,235,030	15,768,179
Net cash provided by (used for) operating activities	172,285,546	101,528,917
Activity in available-for-sale securities:
Sales proceeds	31,899,490	18,968,556
Maturities and calls	6,250,000	31,000,000
Purchases	(51,633,929)	(35,628,408)
Plant and equipment additions	(24,067,012)	(69,424,380)
Proceeds from sale of plant and equipment	15,510	10,803
(Increase) decrease in other assets	(531,456)	(725,542)
Net cash provided by (used for) investing activities	(38,067,397)	(55,798,971)
CASH FLOWS FROM FINANCING ACTIVITIES:
Issuance of common stock from stock plan transactions	16,030,793	7,439,914
Cash dividends paid	(38,701,691)	(36,036,136)
Excess tax benefits from stock-based compensation	1,495,850	586,535
Net cash provided by (used for) financing activities	(21,175,048)	(28,009,687)
NET INCREASE IN CASH AND CASH EQUIVALENTS	113,043,101	17,720,259
CASH AND CASH EQUIVALENTS, beginning of period	389,678,664	357,986,774
CASH AND CASH EQUIVALENTS, end of period	502,721,765	375,707,033
Equity [Abstract]
Common stock issued under stock compensation plan (in shares)	1,190,696
Dividends payable (in dollars per share)	$ 0.14
Dividends payable	$ 20,200,000