Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Stockholders' Equity Attributable to Parent	$ 1,636,003	$ 977,065	$ 1,636,003	$ 977,065	$ 1,104,538	$ 1,073,490	$ 956,059	$ 943,809
Held In Rabbi Trust At Cost [Member]
Stockholders' Equity Attributable to Parent	$ (4,648)	$ (4,653)	$ (4,648)	$ (4,653)	(4,599)	(4,718)	(4,591)	(4,590)
Retained Earnings
Cash dividends declared per common share (in dollars per share)	$ 0.44	$ 0.38	$ 0.88	$ 0.76
Stockholders' Equity Attributable to Parent	$ 585,111	$ 504,926	$ 585,111	$ 504,926	569,582	546,736	484,266	465,937
Deferred Compensation, Share-based Payments [Member]
Stockholders' Equity Attributable to Parent	4,648	4,653	4,648	4,653	4,599	4,718	4,591	4,590
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	20,956	(10,114)	20,956	(10,114)	6,881	(1,173)	(8,195)	(1,831)
Common Stock [Member]
Stockholders' Equity Attributable to Parent	342	274	342	274	280	279	273	273
Additional Paid-in Capital [Member]
Stockholders' Equity Attributable to Parent	$ 1,029,594	$ 481,979	$ 1,029,594	$ 481,979	$ 527,795	$ 527,648	$ 479,715	$ 479,430
Common Class A [Member]
Common Stock, Shares, Outstanding	34,321,061	27,532,524	34,321,061	27,532,524	28,137,504	28,080,408	27,512,328	27,450,190

Aggregate dividends declared during the period for each share of common stock outstanding.

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