Consolidated Statement of Shareholders' Investment - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		280,281,321			280,281,321
Beginning balance		$ 2,049,518,261		$ 1,910,424,234	$ 2,049,518,261	$ 1,910,424,234	$ 1,722,516,761
Issuance of common stock					66,837,820	47,770,467	86,893,439
Repurchases of common stock					(591,577,851)	(231,363,216)	(163,361,221)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					18,305,981	18,376,965	19,192,699
Dividends declared					(117,064,654)	(110,946,799)	(102,268,251)
Net income	$ 106,275,000	$ 111,249,000	$ 130,469,000	$ 97,557,000	437,883,097	406,791,922	347,591,276
Other comprehensive income					$ (2,150,994)	$ 8,464,688	(140,469)
Ending balance (in shares)	259,328,613		280,281,321		259,328,613	280,281,321
Ending balance	$ 1,861,751,660		$ 2,049,518,261		$ 1,861,751,660	$ 2,049,518,261	$ 1,910,424,234
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		280,281,321		287,737,516	280,281,321	287,737,516	291,338,011
Beginning balance		$ 16,816,879		$ 17,264,251	$ 16,816,879	$ 17,264,251	$ 17,480,281
Issuance of common stock (in shares)					5,496,659	4,498,729	6,705,632
Issuance of common stock					$ 329,801	$ 269,923	$ 402,338
Repurchases of common stock (in shares)					(26,449,367)	(11,954,924)	(10,306,127)
Repurchases of common stock					$ (1,586,963)	$ (717,295)	$ (618,368)
Ending balance (in shares)	259,328,613		280,281,321		259,328,613	280,281,321	287,737,516
Ending balance	$ 15,559,717		$ 16,816,879		$ 15,559,717	$ 16,816,879	$ 17,264,251
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		723,510,672		683,446,463	723,510,672	683,446,463	596,782,695
Issuance of common stock					66,508,019	47,500,544	86,491,101
Repurchases of common stock					(63,000,528)	(25,813,300)	(19,020,032)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					18,305,981	18,376,965	19,192,699
Ending balance	745,324,144		723,510,672		745,324,144	723,510,672	683,446,463
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,301,997,327		1,210,984,825	1,301,997,327	1,210,984,825	1,109,384,621
Repurchases of common stock					(526,990,360)	(204,832,621)	(143,722,821)
Impact of ASU 2016-01 adoption					6,642,727
Dividends declared					(117,064,654)	(110,946,799)	(102,268,251)
Net income					437,883,097	406,791,922	347,591,276
Ending balance	1,102,468,137		1,301,997,327		1,102,468,137	1,301,997,327	1,210,984,825
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 7,193,383		$ (1,271,305)	7,193,383	(1,271,305)	(1,130,836)
Impact of ASU 2016-01 adoption					(6,642,727)
Other comprehensive income					(2,150,994)	8,464,688	(140,469)
Ending balance	$ (1,600,338)		$ 7,193,383		$ (1,600,338)	$ 7,193,383	$ (1,271,305)