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, 2014		369,459,000
Shareholders' Equity at Dec. 31, 2014	$ 4,804,954	$ 3,695	$ 3,072,996	$ (690)	$ 1,728,953
Stock option exercises, shares		904,000
Stock option exercises	10,534	$ 9	10,525	0	0
Share issuances, net of cancellations, shares		428,000
Share issuances, net of cancellations	7,424	$ 4	7,420
Dividends declared	117,873		8		117,881
Share repurchases, shares		(21,642,000)
Share repurchases	(442,738)	$ (217)			(442,521)
Share-based compensation	16,888	$ 0	16,888	0	0
Excess tax benefits (deficiencies) from share-based compensation	(14,035)		(14,035)
Net income	494,090				494,090
Other comprehensive income	81			81
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	319,089,720	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	0	0
Cumulative Effect on Retained Earnings, Net of Tax	18,238		(406)		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
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