Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)	6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 166,765,236		$ 151,741,878
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	45,789,787		42,471,227
Gain on disposal of assets	(10,900)		(18,052)
Loss on disposal of assets	705,381		250,247
Gain on sale of investments	(1,657,211)		(4,835,125)
Loss on sale of investments	2,144,668		307,110
Deferred income taxes	(402,154)		(2,472,624)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	9,777,427		11,224,538
Excess tax benefits from stock-based compensation	(1,658,828)		(1,629,160)
Change in operating assets and liabilities:
Accounts receivable, net	(23,473,550)		(28,050,976)
Inventories	(1,531,186)		(9,014,610)
Prepaid expenses and other	19,882,083		11,612,976
Accounts payable	15,469,383		(1,520,783)
Accrued liabilities, excluding dividends declared	21,670,990		21,821,075
Net cash provided by operating activities	253,471,126		191,887,721
Activity in available-for-sale securities:
Sales proceeds	45,982,921		13,805,507
Purchases	(165,496,238)		(12,939,815)
Plant and equipment additions	(54,832,532)		(33,589,833)
Proceeds from sale of plant and equipment	516,667		4
Decrease in other assets	(2,287,732)		(1,084,292)
Net cash used for investing activities	(176,116,914)		(33,808,429)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of long-term debt	(18,750,000)		(3,750,000)
Issuance of common stock from stock plan transactions	29,282,769		13,809,419
Cash dividends paid	(49,334,501)		(47,193,926)
Repurchases of common stock	(92,274,871)		(50,107,196)
Excess tax benefits from stock-based compensation	1,658,828		1,629,160
Net cash used for financing activities	(129,417,775)		(85,612,543)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(52,063,563)		72,466,749
CASH AND CASH EQUIVALENTS, beginning of period	551,557,527	[1]	497,429,804
CASH AND CASH EQUIVALENTS, end of period	$ 499,493,964		$ 569,896,553

[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.