Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2016		Mar. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 80,279,994		$ 77,183,441
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	22,839,030		21,279,105
Gain on disposal of assets	(10,900)		(44,414)
Loss on disposal of assets	577,916		221,953
Gain on sale of investments	(1,498,949)		(2,912,864)
Loss on sale of investments	1,961,673		300,593
Deferred income taxes	(2,403,818)		(15,354)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	4,974,453		5,813,951
Excess tax benefits from stock-based compensation	(427,364)		(1,073,744)
Change in operating assets and liabilities:
Accounts receivable, net	(28,020,070)		(33,773,355)
Inventories	(13,929,365)		(3,806,065)
Prepaid expenses and other	23,521,639		15,219,554
Accounts payable	13,065,406		1,827,677
Accrued liabilities, excluding dividends declared	50,511,519		27,731,368
Net cash provided by operating activities	151,441,164		107,951,846
Activity in available-for-sale securities:
Sales proceeds	43,417,073		7,540,449
Purchases	(25,154,245)		(6,360,397)
Plant and equipment additions	(20,329,035)		(15,160,355)
Proceeds from sale of plant and equipment	516,667		4
Decrease (Increase) in other assets	135,904		(1,150,642)
Net cash provided by (used for) investing activities	(1,413,636)		(15,130,941)
CASH FLOWS (USED FOR) FINANCING ACTIVITIES:
Repayment of long-term debt	(16,875,000)		(1,875,000)
Issuance of common stock from stock plan transactions	10,678,450		9,131,177
Cash dividends paid	(24,759,481)		(23,619,837)
Repurchases of common stock	(44,585,668)		(25,049,145)
Excess tax benefits from stock-based compensation	427,364		1,073,744
Net cash (used for) financing activities	(75,114,335)		(40,339,061)
NET INCREASE IN CASH AND CASH EQUIVALENTS	74,913,193		52,481,844
CASH AND CASH EQUIVALENTS, beginning of period	551,557,527	[1]	497,429,804
CASH AND CASH EQUIVALENTS, end of period	$ 626,470,720		$ 549,911,648

[1] Note: The condensed consolidated balance sheet at December 31, 2015 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements.