Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ 213,239,036	$ 220,272,725
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	53,296,388	55,908,679
(Gain) on disposal of assets	(95,658)	(130,647)
Loss on disposal of assets	340,070	10,544
(Gain) on sale of investments	(218,981)	(1,245,359)
Loss on sale of investments	0	529,484
Deferred income taxes	196,133	(2,743,537)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	10,139,148	8,652,939
Change in operating assets and liabilities:
Accounts receivable, net	(12,888,533)	(8,284,592)
Inventories	187,136	4,389,930
Prepaid expenses and other	(2,311,576)	(9,171,506)
Accounts payable	(2,769,229)	16,885,979
Accrued liabilities, excluding dividends declared and short-term debt	14,353,360	7,309,915
Net cash provided by operating activities	273,467,294	292,384,554
Activity in available-for-sale securities:
Sales proceeds	28,845,319	53,822,336
Maturities and calls	9,714,000	21,884,000
Purchases	(40,298,297)	(177,243,275)
Plant and equipment additions	(45,504,667)	(51,860,448)
Proceeds from sale of plant and equipment	227,738	179,700
Decrease in other assets	(1,022,057)	(2,965,781)
Net cash (used for) investing activities	(48,037,964)	(156,183,468)
CASH FLOWS (USED FOR) FINANCING ACTIVITIES:
Repayment of debt	0	(54,875,000)
Issuance of common stock from stock plan transactions	41,934,814	52,996,593
Cash dividends paid	(57,918,158)	(58,152,113)
Repurchases of common stock	(166,138,372)	(349,205,921)
Net cash (used for) financing activities	(182,121,716)	(409,236,441)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	43,307,614	(273,035,355)
CASH AND CASH EQUIVALENTS, beginning of period	217,025,278	569,734,496
CASH AND CASH EQUIVALENTS, end of period	$ 260,332,892	$ 296,699,141