Consolidated Statements Of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net Income	$ 168,586,840	$ 164,668,228	$ 137,733,969
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	50,179,823	42,634,684	38,620,674
Gain on disposal of assets	(419,382)	0	0
Loss on disposal of assets	952,190	992,876	1,070,484
Gain on sale of investments	(9,584,288)	(15,787,691)	(9,767,008)
Loss on sale of investments	1,216,027	9,134,183	2,398,483
Deferred income taxes	843,484	13,942,674	10,390,886
Stock based compensation expense related to employee stock options, employee stock purchases and restricted stock	16,433,837	14,197,557	10,652,196
Excess tax benefits from stock based compensation	(815,651)	(5,362,501)	(3,020,975)
Change in operating assets and liabilities:
Accounts receivable	810,022	(14,742,103)	(24,488,100)
Inventories	28,823,046	(88,024,582)	(47,119,734)
Prepaid expenses and other	14,868,742	(6,954,317)	(1,353,238)
Accounts payable	(22,270,571)	25,175,109	12,838,717
Accrued liabilities	8,221,935	1,794,548	177,332
Net cash provided by operating activities	257,846,054	141,668,665	128,133,686
Activity in available-for-sale securities:
Sales proceeds	38,831,234	107,763,498	56,421,319
Maturities and calls	75,652,000	104,375,000	36,000,000
Purchases	(112,545,415)	(196,365,327)	(164,844,351)
Plant and equipment additions	(117,474,449)	(120,177,730)	(46,896,950)
Proceeds from sale of plant and equipment	1,064,614	179,212	504,977
Decrease (increase) in other assets	(17,602,228)	698,357	(5,771,488)
Net cash provided by (used for) investing activities	(132,074,244)	(103,526,990)	(124,586,493)
CASH FLOWS FROM FINANCING ACTIVITIES:
Issuance of common stock from stock plan transactions	12,208,881	33,304,764	67,067,391
Cash dividends paid	(73,387,727)	(67,171,939)	(61,394,232)
Repurchases of common stock	(33,716,725)	0	0
Excess tax benefits from stock based compensation	815,651	5,362,501	3,020,975
Net cash used for (provided by) financing activities	(94,079,920)	(28,504,674)	8,694,134
NET INCREASE IN CASH AND CASH EQUIVALENTS	31,691,890	9,637,001	12,241,327
CASH AND CASH EQUIVALENTS, Beginning of year	357,986,774	348,349,773	336,108,446
CASH AND CASH EQUIVALENTS, End of year	$ 389,678,664	$ 357,986,774	$ 348,349,773