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	123,539			123,539
Other comprehensive (loss) income	53,593				53,593
Dividends declared on common stock	(30,373)			(30,373)
Stock compensation expense	11,052		11,052
Net issuance of treasury stock for vested restricted stock units	(4,123)		(1,287)			(2,836)
Net issuance of treasury stock for vested restricted stock units (in shares)		180
Adjustments to Additional Paid in Capital, Stock Split		$ 875	(875)
Stock Issued During Period, Shares, Stock Splits		55,142
Ending Balance at Sep. 30, 2017	623,103	$ 1,751	258,653	1,651,760	(170,263)	(1,118,798)
Ending Balance (in shares) at Sep. 30, 2017		110,373
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	185,802			185,802
Other comprehensive (loss) income	(23,283)				(23,283)
Dividends declared on common stock	(33,833)			(33,833)
Stock compensation expense	11,162		11,162
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 Sep. 30, 2018	$ 911,917	$ 1,751	$ 272,301	$ 1,970,875	$ (212,256)	$ (1,120,754)
Ending Balance (in shares) at Sep. 30, 2018		110,618