Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Stockholders' Equity Attributable to Parent	$ 1,671,692	$ 1,636,003	$ 1,671,692	$ 1,636,003	$ 1,679,656	$ 1,708,143	$ 1,104,538	$ 1,073,490
Held In Rabbi Trust At Cost [Member]
Stockholders' Equity Attributable to Parent	$ (4,649)	$ (4,648)	$ (4,649)	$ (4,648)	(4,604)	(4,735)	(4,599)	(4,718)
Retained Earnings
Cash dividends declared per common share (in dollars per share)	$ 0.46	$ 0.44	$ 0.92	$ 0.88
Stockholders' Equity Attributable to Parent	$ 676,834	$ 585,111	$ 676,834	$ 585,111	667,084	654,182	569,582	546,736
Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity Attributable to Parent	4,649	4,648	4,649	4,648	4,604	4,735	4,599	4,718
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	51,845	20,956	51,845	20,956	49,728	18,169	6,881	(1,173)
Common Stock [Member]
Stockholders' Equity Attributable to Parent	328	342	328	342	331	342	280	279
Additional Paid-in Capital [Member]
Stockholders' Equity Attributable to Parent	$ 942,685	$ 1,029,594	$ 942,685	$ 1,029,594	$ 962,513	$ 1,035,450	$ 527,795	$ 527,648
Common Class A [Member]
Common Stock, Shares, Outstanding	32,942,110	34,321,061	32,942,110	34,321,061	33,260,005	34,377,388	28,137,504	28,080,408