Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Common Stock, Shares, Outstanding	33,043,812		33,043,812			32,965,692
Stockholders' Equity Attributable to Parent	$ 1,755,954	$ 1,689,724	$ 1,755,954	$ 1,689,724	$ 1,741,622	$ 1,702,685	$ 1,671,692	$ 1,708,143
Held In Rabbi Trust At Cost [Member]
Stockholders' Equity Attributable to Parent	$ (3,157)	$ (4,712)	$ (3,157)	$ (4,712)	(3,116)	(3,066)	(4,649)	(4,735)
Retained Earnings
Cash dividends declared per common share (in dollars per share)	$ 0.48	$ 0.46	$ 1.44	$ 1.38
Stockholders' Equity Attributable to Parent	$ 787,742	$ 696,546	$ 787,742	$ 696,546	763,596	716,024	676,834	654,182
Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity Attributable to Parent	3,157	4,712	3,157	4,712	3,116	3,066	4,649	4,735
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	18,567	48,632	18,567	48,632	29,567	40,695	51,845	18,169
Common Stock [Member]
Stockholders' Equity Attributable to Parent	329	328	329	328	329	328	328	342
Additional Paid-in Capital [Member]
Stockholders' Equity Attributable to Parent	$ 949,316	$ 944,218	$ 949,316	$ 944,218	$ 948,130	$ 945,638	$ 942,685	$ 1,035,450
Common Class A [Member]
Common Stock, Shares, Outstanding	33,043,812	32,955,547	33,043,812	32,955,547	33,037,859	32,965,692	32,942,110	34,377,388