Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net Income	$ 331,608,542		$ 276,323,023
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	80,748,272		74,858,551
Gain on disposal of assets	(145,777)		(163,984)
Loss on disposal of assets	26,839		288,323
Gain on sale of investments	(1,508,411)		(1,028,505)
Loss on sale of investments	532,494		113,091
Deferred income taxes	(5,538,048)		6,855,507
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	13,558,557		13,090,804
Change in operating assets and liabilities:
Accounts receivable, net	(16,200,315)		(28,914,071)
Inventories	3,371,546		(17,346,380)
Prepaid expenses and other	(14,797,969)		14,540,408
Accounts payable	143,514		(60,096)
Accrued liabilities, excluding dividends declared and short-term debt	6,408,044		13,999,423
Net cash provided by operating activities	398,207,288		352,556,094
Activity in available-for-sale securities:
Sales proceeds	54,078,232		2,888,493
Maturities and calls	51,892,136		18,100,000
Purchases	(180,657,259)		(18,062,164)
Plant and equipment additions	(68,771,193)		(86,111,836)
Proceeds from sale of plant and equipment	194,200		192,756
Decrease (increase) in other assets	(4,557,125)		2,432,320
Net cash used for investing activities	(147,821,009)		(80,560,431)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repayment of debt	(78,000,000)		(78,125,000)
Issuance of common stock from stock plan transactions	61,379,666		31,896,752
Cash dividends paid	(87,733,776)		(80,417,057)
Repurchases of common stock	(521,749,624)		(129,827,666)
Net cash used for financing activities	(626,103,734)		(256,472,971)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(375,717,455)		15,522,692
CASH AND CASH EQUIVALENTS, beginning of period	569,734,496	[1]	546,477,075
CASH AND CASH EQUIVALENTS, end of period	$ 194,017,041		$ 561,999,767

[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.