Unaudited Condensed Consolidated Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 104,280,411	$ 111,248,743
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	28,098,804	28,046,434
(Gain) on disposal of assets	(43,338)	(4,374)
Loss on disposal of assets	122,296	8,677
(Gain) on sale of investments	(88,576)	(1,245,075)
Loss on sale of investments	0	529,484
Deferred income taxes	(1,233,853)	(2,746,797)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	4,862,947	3,977,471
Change in operating assets and liabilities:
Accounts receivable, net	(30,728,497)	(15,305,451)
Inventories	(21,443)	9,532,631
Prepaid expenses and other	10,847,705	2,872,873
Accounts payable	(2,692,820)	(12,590,570)
Accrued liabilities, excluding dividends declared and short-term debt	20,436,582	23,125,002
Net cash provided by operating activities	133,840,218	147,449,048
Activity in available-for-sale securities:
Sales proceeds	17,139,689	53,822,052
Maturities and calls	3,000,000	3,000,000
Purchases	(18,475,158)	(1,213,405)
Plant and equipment additions	(16,844,301)	(26,247,890)
Proceeds from sale of plant and equipment	117,455	67,400
Decrease in other assets	164,215	45,322
Net cash (used for) provided by investing activities	(14,898,100)	29,473,479
CASH FLOWS (USED FOR) FINANCING ACTIVITIES:
Repayment of debt	0	(28,000,000)
Issuance of common stock from stock plan transactions	10,514,554	36,267,047
Cash dividends paid	(28,526,146)	(28,028,132)
Repurchases of common stock	(96,267,229)	(202,572,378)
Net cash (used for) financing activities	(114,278,821)	(222,333,463)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	4,663,297	(45,410,936)
CASH AND CASH EQUIVALENTS, beginning of period	217,025,278	569,734,496
CASH AND CASH EQUIVALENTS, end of period	$ 221,688,575	$ 524,323,560