Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Common Stock, Shares, Outstanding	33,037,859		33,037,859			32,965,692
Stockholders' Equity Attributable to Parent	$ 1,741,622	$ 1,671,692	$ 1,741,622	$ 1,671,692	$ 1,715,371	$ 1,702,685	$ 1,679,656	$ 1,708,143
Held In Rabbi Trust At Cost [Member]
Stockholders' Equity Attributable to Parent	$ (3,116)	$ (4,649)	$ (3,116)	$ (4,649)	(3,080)	(3,066)	(4,604)	(4,735)
Retained Earnings
Cash dividends declared per common share (in dollars per share)	$ 0.48	$ 0.46	$ 0.96	$ 0.92
Stockholders' Equity Attributable to Parent	$ 763,596	$ 676,834	$ 763,596	$ 676,834	741,883	716,024	667,084	654,182
Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity Attributable to Parent	3,116	4,649	3,116	4,649	3,080	3,066	4,604	4,735
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	29,567	51,845	29,567	51,845	27,157	40,695	49,728	18,169
Common Stock [Member]
Stockholders' Equity Attributable to Parent	329	328	329	328	329	328	331	342
Additional Paid-in Capital [Member]
Stockholders' Equity Attributable to Parent	$ 948,130	$ 942,685	$ 948,130	$ 942,685	$ 946,002	$ 945,638	$ 962,513	$ 1,035,450
Common Class A [Member]
Common Stock, Shares, Outstanding	33,037,859	32,942,110	33,037,859	32,942,110	33,024,882	32,965,692	33,260,005	34,377,388