Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance, shares at Dec. 31, 2014		220,161,120
Balance, value at Dec. 31, 2014	$ 3,777,626	$ 3,519,316	$ 246,242	$ 12,068
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	222,539		222,539
Other comprehensive income (loss), net of tax	(14,626)			(14,626)
Stock-based compensation	14,383	$ 14,383
Stock repurchased and retired, shares		(844,215)
Stock repurchased and retired	(14,589)	$ (14,589)
Issuances of common stock under stock plans, shares		854,186
Issuances of common stock under stock plans	1,481	$ 1,481
Cash dividends on common stock	(137,480)		(137,480)
Balance, shares at Dec. 31, 2015		220,171,091
Balance, value at Dec. 31, 2015	3,849,334	$ 3,520,591	331,301	(2,558)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	232,940		232,940
Other comprehensive income (loss), net of tax	(18,785)			(18,785)
Stock-based compensation	9,790	$ 9,790
Stock repurchased and retired, shares		(1,117,061)
Stock repurchased and retired	(17,708)	$ (17,708)
Issuances of common stock under stock plans, shares		1,123,000
Issuances of common stock under stock plans	2,626	$ 2,626
Cash dividends on common stock	$ (141,402)		(141,402)
Balance, shares at Dec. 31, 2016	220,177,030	220,177,030
Balance, value at Dec. 31, 2016	$ 3,916,795	$ 3,515,299	422,839	(21,343)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	246,019		246,019
Other comprehensive income (loss), net of tax	781			781
Stock-based compensation	9,612	$ 9,612
Stock repurchased and retired, shares		(468,555)
Stock repurchased and retired	(8,614)	$ (8,614)
Issuances of common stock under stock plans, shares		440,349
Issuances of common stock under stock plans	961	$ 961
Cash dividends on common stock	(150,768)		(150,768)
Tax rate effect reclassification	$ 0		4,430	(4,430)
Balance, shares at Dec. 31, 2017	220,148,824	220,148,824
Balance, value at Dec. 31, 2017	$ 4,014,786	$ 3,517,258	$ 522,520	$ (24,992)