Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net Income		$ 111,248,743	$ 97,556,741
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		28,046,434	25,181,854
Gain on disposal of assets		(4,374)	(146,261)
Loss on disposal of assets		8,677	359,959
Gain on sale of investments		(1,245,075)	(274,844)
Loss on sale of investments		529,484	21,431
Deferred income taxes		(2,746,797)	5,718,363
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock		3,977,471	4,422,996
Change in operating assets and liabilities:
Accounts receivable, net		(15,305,451)	(37,866,173)
Inventories		9,532,631	(7,776,814)
Prepaid expenses and other		2,872,873	18,857,501
Accounts payable		(12,590,570)	(2,249,350)
Accrued liabilities, excluding dividends declared and short-term debt		23,125,002	27,385,768
Net cash provided by operating activities		147,449,048	131,191,171
Activity in available-for-sale securities:
Sales proceeds		53,822,052	1,316,280
Maturities and calls		3,000,000	6,100,000
Purchases		(1,213,405)	(16,225,018)
Plant and equipment additions		(26,247,890)	(27,119,993)
Proceeds from sale of plant and equipment		67,400	15,001
Decrease (Increase) in other assets		45,322	(588,361)
Net cash provided by (used for) investing activities		29,473,479	(36,502,091)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of debt		(28,000,000)	(41,875,000)
Issuance of common stock from stock plan transactions		36,267,047	17,237,232
Cash dividends paid		(28,028,132)	(25,896,377)
Repurchases of common stock		(202,572,378)	(30,986,385)
Net cash used for financing activities		(222,333,463)	(81,520,530)
NET INCREASE IN CASH AND CASH EQUIVALENTS		(45,410,936)	13,168,550
CASH AND CASH EQUIVALENTS, beginning of period	[1]	569,734,496
CASH AND CASH EQUIVALENTS, end of period		$ 524,323,560	$ 559,645,625

[1] The condensed consolidated balance sheet at December 31, 2017 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.