Condensed Consolidated Statements Of Cash Flows (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 45,430,567	$ 46,344,584
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	13,805,949	12,674,712
(Gain)/loss on disposal of assets	1,495,536	161,487
(Gain) loss on sale of investments	(2,822,207)	(1,494,565)
Deferred income taxes	(2,180,428)	880,311
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	3,850,063	3,960,579
Excess tax benefits from stock-based compensation	(341,225)	(318,115)
Change in operating assets and liabilities:
Accounts receivable, net	(12,347,433)	(28,907,130)
Inventories	29,860,576	(11,997,848)
Prepaid expenses and other	133,880	11,876,847
Accounts payable	(10,198,080)	7,515,474
Accrued liabilities, excluding dividends declared	30,735,336	20,435,185
Net cash provided by (used for) operating activities	97,422,534	61,131,521
Activity in available-for-sale securities:
Sales proceeds	10,627,243	6,527,702
Maturities and calls	3,000,000	17,000,000
Purchases	(33,136,682)	(17,687,907)
Plant and equipment additions	(12,674,970)	(39,824,858)
Proceeds from sale of plant and equipment	9,500	0
(Increase) decrease in other assets	(706,899)	175,419
Net cash provided by (used for) investing activities	(32,881,808)	(33,809,644)
CASH FLOWS FROM FINANCING ACTIVITIES:
Issuance of common stock from stock plan transactions	6,592,272	2,466,307
Cash dividends paid	(18,599,926)	(17,288,348)
Repurchases of common stock	0	0
Excess tax benefits from stock-based compensation	341,225	318,115
Net cash provided by (used for) financing activities	(11,666,429)	(14,503,926)
NET INCREASE IN CASH AND CASH EQUIVALENTS	52,874,297	12,817,951
CASH AND CASH EQUIVALENTS, beginning of period	389,678,664	357,986,774
CASH AND CASH EQUIVALENTS, end of period	$ 442,552,961	$ 370,804,725