Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		259,328,613			259,328,613
Beginning balance		$ 1,861,751,660		$ 2,049,518,261	$ 1,861,751,660	$ 2,049,518,261	$ 1,910,424,234
Issuance of common stock					77,821,151	66,837,820	47,770,467
Repurchases of common stock					(331,471,392)	(591,577,851)	(231,363,216)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					21,671,192	18,305,981	18,376,965
Impact of ASU 2016-01 adoption						0
Dividends declared					(116,679,965)	(117,064,654)	(110,946,799)
Net income	$ 99,547,000	$ 104,280,000	$ 106,275,000	$ 111,249,000	424,683,939	437,883,097	406,791,922
Other comprehensive income					$ 311,235	$ (2,150,994)	8,464,688
Ending balance (in shares)	251,277,515		259,328,613		251,277,515	259,328,613
Ending balance	$ 1,938,087,820		$ 1,861,751,660		$ 1,938,087,820	$ 1,861,751,660	$ 2,049,518,261
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		259,328,613		280,281,321	259,328,613	280,281,321	287,737,516
Beginning balance		$ 15,559,717		$ 16,816,879	$ 15,559,717	$ 16,816,879	$ 17,264,251
Issuance of common stock (in shares)					5,724,840	5,496,659	4,498,729
Issuance of common stock					$ 343,490	$ 329,801	$ 269,923
Repurchases of common stock (in shares)					(13,775,938)	(26,449,367)	(11,954,924)
Repurchases of common stock					$ (826,556)	$ (1,586,963)	$ (717,295)
Ending balance (in shares)	251,277,515		259,328,613		251,277,515	259,328,613	280,281,321
Ending balance	$ 15,076,651		$ 15,559,717		$ 15,076,651	$ 15,559,717	$ 16,816,879
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		745,324,144		723,510,672	745,324,144	723,510,672	683,446,463
Issuance of common stock					77,477,661	66,508,019	47,500,544
Repurchases of common stock					(36,544,858)	(63,000,528)	(25,813,300)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					21,671,192	18,305,981	18,376,965
Ending balance	807,928,139		745,324,144		807,928,139	745,324,144	723,510,672
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,102,468,137		1,301,997,327	1,102,468,137	1,301,997,327	1,210,984,825
Repurchases of common stock					(294,099,978)	(526,990,360)	(204,832,621)
Impact of ASU 2016-01 adoption						6,642,727
Dividends declared					(116,679,965)	(117,064,654)	(110,946,799)
Net income					424,683,939	437,883,097	406,791,922
Ending balance	1,116,372,133		1,102,468,137		1,116,372,133	1,102,468,137	1,301,997,327
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (1,600,338)		$ 7,193,383	(1,600,338)	7,193,383	(1,271,305)
Impact of ASU 2016-01 adoption						(6,642,727)
Other comprehensive income					311,235	(2,150,994)	8,464,688
Ending balance	$ (1,289,103)		$ (1,600,338)		$ (1,289,103)	$ (1,600,338)	$ 7,193,383