Unaudited Condensed Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	$ 1,904,334,235	$ 1,921,499,312	$ 1,861,751,660	[1]	$ 2,049,518,261
Issuance of common stock and the tax benefit of stock plan transactions	26,287,061	8,383,073	$ 68,221,875		61,379,666
Repurchases of common stock (in shares)			(11,400,000)
Repurchases of common stock	(96,625,003)	(172,543,703)	$ (262,763,375)		(521,749,624)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	5,788,286	4,905,626	15,927,434		13,558,565
Impact of ASU 2016-01 adoption					0
Dividends declared	(29,092,442)	(28,832,984)	(87,784,052)		(88,538,509)
Net income (loss)	111,897,852	111,335,817	325,136,888		331,608,542
Other comprehensive income (loss)	(1,261,726)	(1,642,886)	837,833		(2,672,647)
Balance at end of period	$ 1,921,328,263	$ 1,843,104,255	$ 1,921,328,263		$ 1,843,104,255
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	254,779,119	268,923,121	259,328,613		280,281,321
Balance at beginning of period	$ 15,286,747	$ 16,135,387	$ 15,559,717		$ 16,816,879
Issuance of common stock and the tax benefit of stock plan transactions (in shares)	1,771,463	702,296	5,021,616		4,960,412
Issuance of common stock and the tax benefit of stock plan transactions	$ 106,287	$ 42,138	$ 301,297		$ 297,625
Repurchases of common stock (in shares)	(3,569,577)	(7,508,249)	(11,369,224)		(23,124,565)
Repurchases of common stock	$ (214,174)	$ (450,495)	$ (682,154)		$ (1,387,474)
Ending balance (in shares)	252,981,005	262,117,168	252,981,005		262,117,168
Balance at end of period	$ 15,178,860	$ 15,727,030	$ 15,178,860		$ 15,727,030
Additional  Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	777,238,253	748,794,815	745,324,144		723,510,672
Issuance of common stock and the tax benefit of stock plan transactions	26,180,774	8,340,935	67,920,578		61,082,042
Repurchases of common stock	(9,744,945)	(18,545,378)	(29,709,788)		(54,655,280)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	5,788,286	4,905,626	15,927,434		13,558,565
Balance at end of period	799,462,368	743,495,999	799,462,368		743,495,999
Retained  Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	1,111,310,014	1,157,048,214	1,102,468,137		1,301,997,327
Repurchases of common stock	(86,665,884)	(153,547,831)	(232,371,433)		(465,706,871)
Impact of ASU 2016-01 adoption					6,642,727
Dividends declared	(29,092,442)	(28,832,984)	(87,784,052)		(88,538,509)
Net income (loss)	111,897,852	111,335,817	325,136,888		331,608,542
Balance at end of period	1,107,449,540	1,086,003,216	1,107,449,540		1,086,003,216
Accumulated  Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	499,221	(479,104)	(1,600,338)		7,193,383
Impact of ASU 2016-01 adoption					(6,642,727)
Other comprehensive income (loss)	(1,261,726)	(1,642,886)	837,833		(2,672,645)
Balance at end of period	$ (762,505)	$ (2,121,990)	$ (762,505)		$ (2,121,990)

[1] The condensed consolidated balance sheet at December 31, 2018 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.