Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common stock security	Paid-in Capital	Accumulated Other Comprehensive Earnings (Loss)	Retained Earnings	Deferred Compensation	Treasury Stock at Cost	Total
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2014	$ 66,033	$ 213,737	$ 171,383	$ 786,908	$ 13,769	$ (406,768)	$ 845,062
Balance (in shares) at Dec. 31, 2014	43,102,715
Increase (Decrease) in Shareholders' Equity
Net earnings				137,544			137,544
Other comprehensive earnings (loss), net of tax			(47,609)				(47,609)
Deferred compensation under Rabbi trust plans					(3,122)	3,122
Stock option excess tax benefit		11,413					11,413
Exercise of stock options	$ 441	(3,805)					(3,364)
Exercise of stock options (in shares)	441,413
Dividends paid ($2.58, $2.79 and $2.75 per share during 2017, 2016 and 2015, respectively)				(119,577)			(119,577)
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2015	$ 66,474	221,345	123,774	804,875	10,647	(403,646)	823,469
Balance (in shares) at Dec. 31, 2015	43,544,128
Increase (Decrease) in Shareholders' Equity
Net earnings				114,920			114,920
Other comprehensive earnings (loss), net of tax			(1,164)				(1,164)
Deferred compensation under Rabbi trust plans					849	(849)
Stock option excess tax benefit		9,576					9,576
Exercise of stock options	$ 401	(1,142)					(741)
Exercise of stock options (in shares)	400,569
Dividends paid ($2.58, $2.79 and $2.75 per share during 2017, 2016 and 2015, respectively)				(122,488)			(122,488)
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2016	$ 66,875	229,779	122,610	797,307	11,496	(404,495)	823,572
Balance (in shares) at Dec. 31, 2016	43,944,697
Increase (Decrease) in Shareholders' Equity
Net earnings				105,028			105,028
Other comprehensive earnings (loss), net of tax			35,309				35,309
Deferred compensation under Rabbi trust plans					(2,856)	2,856
Exercise of stock options	$ 204	3,298					3,502
Exercise of stock options (in shares)	203,658
Dividends paid ($2.58, $2.79 and $2.75 per share during 2017, 2016 and 2015, respectively)				(113,813)			(113,813)
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2017	$ 67,079	$ 233,077	$ 157,919	$ 788,522	$ 8,640	$ (401,639)	$ 853,598
Balance (in shares) at Dec. 31, 2017	44,148,355