Consolidated Statements Of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Shareholders' Equity, shares beginning of period (shares) at Dec. 31, 2016		319,090
Shareholders' Equity beginning of period at Dec. 31, 2016	$ 4,659,363	$ 3,191	$ 3,116,490	$ (526)	$ 1,540,208
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change (see Note 1)	18,237		(406)		18,643
Stock option exercises (shares)		1,954
Stock option exercises	22,765	$ 20	22,745
Stock issuances, net of cancellations (shares)		741
Stock issuances, net of cancellations	3,565	$ 10	3,555
Dividends declared	(83,685)				(83,685)
Share repurchases (shares)		(27,849)
Share repurchases	(665,812)	$ (281)			(665,531)
Share-based compensation	20,784		20,784
Net income	369,807				369,807
Other comprehensive income	61			61
Shareholders' Equity, shares end of period (shares) at Sep. 30, 2017		293,936
Shareholders' Equity end of period at Sep. 30, 2017	4,345,085	$ 2,940	3,163,168	(465)	1,179,442
Shareholders' Equity, shares beginning of period (shares) at Dec. 31, 2017		286,752
Shareholders' Equity beginning of period at Dec. 31, 2017	4,154,026	$ 2,868	3,171,542	(445)	980,061
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change (see Note 1)	22,411		0
Stock option exercises (shares)		514
Stock option exercises	5,462	$ 5	5,457
Stock issuances, net of cancellations (shares)		874
Stock issuances, net of cancellations	3,483	$ 9	3,474
Dividends declared	(77,673)				(77,673)
Share repurchases (shares)		(6,073)
Share repurchases	(179,439)	$ (61)	(284)		(179,094)
Share-based compensation	16,158		16,158
Net income	784,375				784,375
Other comprehensive income	75			75
Shareholders' Equity, shares end of period (shares) at Sep. 30, 2018		282,067
Shareholders' Equity end of period at Sep. 30, 2018	$ 4,728,878	$ 2,821	$ 3,196,347	$ (370)	$ 1,530,080