Consolidated Statements Of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Shareholders' Equity, shares at Dec. 31, 2015		349,149,000
Shareholders' Equity at Dec. 31, 2015	$ 4,759,325	$ 3,491	$ 3,093,802	$ (609)	$ 1,662,641
Stock option exercises, shares		498,000
Stock option exercises	5,845	$ 5	5,840	0	0
Share issuances, net of cancellations, shares		530,000
Share issuances, net of cancellations	8,856	$ 5	8,851
Dividends declared	(122,240)		0		(122,240)
Share repurchases, shares		(31,087,000)
Share repurchases	(603,206)	$ (310)			(602,896)
Share-based compensation	18,626	$ 0	18,626	0	0
Excess tax benefits (deficiencies) from share-based compensation	(10,629)		(10,629)
Net income	602,703				602,703
Other comprehensive income	83			83
Shareholders' Equity, shares at Dec. 31, 2016		319,090,000
Shareholders' Equity at Dec. 31, 2016	4,659,363	$ 3,191	3,116,490	(526)	1,540,208
Stock option exercises, shares		2,352,000
Stock option exercises	27,720	$ 24	27,696
Cumulative Effect on Retained Earnings, Net of Tax	18,238	$ 0	(406)	0	18,644
Share issuances, net of cancellations, shares		730,000
Share issuances, net of cancellations	3,565	$ 10	3,555
Dividends declared	(110,046)		0		(110,046)
Share repurchases, shares		(35,420,000)
Share repurchases	(916,323)	$ (357)			(915,966)
Share-based compensation	24,207	$ 0	24,207	0	0
Excess tax benefits (deficiencies) from share-based compensation	0		0
Net income	447,221				447,221
Other comprehensive income	$ 81			81
Shareholders' Equity, shares at Dec. 31, 2017	286,752,436	286,752,000
Shareholders' Equity at Dec. 31, 2017	$ 4,154,026	$ 2,868	3,171,542	(445)	980,061
Stock option exercises, shares		605,000
Stock option exercises	6,555	$ 6	6,549	0	0
Cumulative Effect on Retained Earnings, Net of Tax	22,411	$ 0			22,411
Share issuances, net of cancellations, shares		935,000
Share issuances, net of cancellations	3,484	$ 9	3,475
Dividends declared	(108,489)		0		(108,489)
Share repurchases, shares		(11,182,000)
Share repurchases	(302,473)	$ (112)	(284)		(302,077)
Share-based compensation	20,145	$ 0	20,145	0	0
Net income	1,022,023				1,022,023
Other comprehensive income	$ 100			100
Shareholders' Equity, shares at Dec. 31, 2018	277,109,507	277,110,000
Shareholders' Equity at Dec. 31, 2018	$ 4,817,782	$ 2,771	$ 3,201,427	$ (345)	$ 1,613,929