Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ 325,136,888	$ 331,608,542
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	79,331,513	80,748,272
(Gain) on disposal of assets	(148,376)	(145,777)
Loss on disposal of assets	495,697	26,839
(Gain) on sale of investments	(593,351)	(1,508,411)
Loss on sale of investments	0	532,494
Change in deferred income taxes	2,189,818	(5,538,048)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	15,927,434	13,558,557
Change in operating assets and liabilities:
Accounts receivable, net	(39,570,916)	(16,200,315)
Inventories	(13,395,196)	3,371,546
Prepaid expenses and other	(2,729,945)	(14,797,969)
Accounts payable	2,499,481	143,514
Accrued liabilities, excluding dividends declared and short-term debt	14,797,251	6,408,044
Net cash provided by operating activities	383,940,298	398,207,288
Activity in available-for-sale securities:
Sales proceeds	57,087,935	54,078,232
Maturities and calls	9,714,000	51,892,136
Purchases	(66,360,636)	(180,657,259)
Plant and equipment additions	(58,322,472)	(68,771,193)
Proceeds from sale of plant and equipment	1,937,207	194,200
Decrease in other assets	(3,111,644)	(4,557,125)
Net cash (used for) investing activities	(59,055,610)	(147,821,009)
CASH FLOWS (USED FOR) FINANCING ACTIVITIES:
Repayment of debt	0	(78,000,000)
Issuance of common stock from stock plan transactions	68,221,875	61,379,666
Cash dividends paid	(87,217,383)	(87,733,776)
Repurchases of common stock	(262,763,375)	(521,749,624)
Net cash (used for) financing activities	(281,758,883)	(626,103,734)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	43,125,805	(375,717,455)
CASH AND CASH EQUIVALENTS, beginning of period	217,025,278	569,734,496
CASH AND CASH EQUIVALENTS, end of period	$ 260,151,083	$ 194,017,041