Condensed Consolidated Statements Of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock
Beginning Balance at Dec. 31, 2016	$ 469,415	$ 876	$ 249,763	$ 1,558,594	$ (223,856)	$ (1,115,962)
Beginning Balance (in shares) at Dec. 31, 2016		55,051
Increase (Decrease) in Stockholders' Equity
Net income	51,157			51,157
Other comprehensive (loss) income	38,717				38,717
Dividends declared on common stock	(20,253)			(20,253)
Stock compensation expense	7,202		7,202
Net issuance of treasury stock for vested restricted stock units	(3,231)		(1,013)			(2,218)
Net issuance of treasury stock for vested restricted stock units (in shares)		123
Adjustments to Additional Paid in Capital, Stock Split		$ 875	(875)
Stock Issued During Period, Shares, Stock Splits		55,142
Ending Balance at Jun. 30, 2017	543,007	$ 1,751	255,077	1,589,498	(185,139)	(1,118,180)
Ending Balance (in shares) at Jun. 30, 2017		110,316
Beginning Balance at Dec. 31, 2017	766,065	$ 1,751	262,201	1,809,845	(188,973)	(1,118,759)
Beginning Balance (in shares) at Dec. 31, 2017		110,385
Increase (Decrease) in Stockholders' Equity
Adoption of accounting standards update for revenue recognition	9,061			9,061
Net income	101,067			101,067
Other comprehensive (loss) income	(19,990)				(19,990)
Dividends declared on common stock	(22,556)			(22,556)
Stock compensation expense	7,420		7,420
Net issuance of treasury stock for vested restricted stock units	(3,057)		(1,062)			(1,995)
Net issuance of treasury stock for vested restricted stock units (in shares)		233
Ending Balance at Jun. 30, 2018	$ 838,010	$ 1,751	$ 268,559	$ 1,897,417	$ (208,963)	$ (1,120,754)
Ending Balance (in shares) at Jun. 30, 2018		110,618