Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Common Stock, Shares, Outstanding	32,955,547		32,955,547			34,377,388
Stockholders' Equity Attributable to Parent	$ 1,689,724	$ 1,682,324	$ 1,689,724	$ 1,682,324	$ 1,671,692	$ 1,708,143	$ 1,636,003	$ 1,073,490
Held In Rabbi Trust At Cost [Member]
Stockholders' Equity Attributable to Parent	$ (4,712)	$ (4,713)	$ (4,712)	$ (4,713)	(4,649)	(4,735)	(4,648)	(4,718)
Retained Earnings
Cash dividends declared per common share (in dollars per share)	$ 0.46	$ 0.44	$ 1.38	$ 1.32
Stockholders' Equity Attributable to Parent	$ 696,546	$ 621,831	$ 696,546	$ 621,831	676,834	654,182	585,111	546,736
Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity Attributable to Parent	4,712	4,713	4,712	4,713	4,649	4,735	4,648	4,718
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	48,632	26,202	48,632	26,202	51,845	18,169	20,956	(1,173)
Common Stock [Member]
Stockholders' Equity Attributable to Parent	328	342	328	342	328	342	342	279
Additional Paid-in Capital [Member]
Stockholders' Equity Attributable to Parent	$ 944,218	$ 1,033,949	$ 944,218	$ 1,033,949	$ 942,685	$ 1,035,450	$ 1,029,594	$ 527,648
Common Class A [Member]
Common Stock, Shares, Outstanding	32,955,547	34,366,781	32,955,547	34,366,781	32,942,110	34,377,388	34,321,061	28,080,408