Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated other comprehensive loss	Treasury Stock	Total
Balance at Dec. 31, 2016	$ 1,395	$ 2,484,251	$ 78,850	$ (88,011)		$ 2,476,485
Balance (in shares) at Dec. 31, 2016	139,530,654
Increase (Decrease) in Stockholders' Equity
Net income			56,740			56,740
Cash dividends declared ($0.24 and $0.22 per share for three months ended March 31, 2018 and 2017, respectively)			(30,700)			(30,700)
Common stock issued under Employee Stock Purchase Plan		528				528
Common stock issued under Employee Stock Purchase Plan (in shares)	15,961
Equity-based awards		1,817	(195)			1,622
Other comprehensive (loss) income, net of tax				1,319		1,319
Balance at Mar. 31, 2017	$ 1,395	2,486,596	104,695	(86,692)		2,505,994
Balance (in shares) at Mar. 31, 2017	139,546,615
Balance at Dec. 31, 2017	$ 1,396	2,488,643	139,177	(96,383)	$ (282)	$ 2,532,551
Balance (in shares) at Dec. 31, 2017	139,588,782					139,588,782
Increase (Decrease) in Stockholders' Equity
Net income			67,958			$ 67,958
Cash dividends declared ($0.24 and $0.22 per share for three months ended March 31, 2018 and 2017, respectively)			(33,504)			(33,504)
Common stock issued under Employee Stock Purchase Plan		342				342
Common stock issued under Employee Stock Purchase Plan (in shares)	12,341
Equity-based awards		1,925	(177)			1,748
Other comprehensive (loss) income, net of tax				(48,233)		(48,233)
Balance at Mar. 31, 2018	$ 1,396	$ 2,490,910	193,522	(164,684)	$ (282)	$ 2,520,862
Balance (in shares) at Mar. 31, 2018	139,601,123					139,601,123
Increase (Decrease) in Stockholders' Equity
Adoption of ASU | ASU 2018-02			$ 20,068	$ (20,068)