Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands		Total		Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Deferred Compensation [Member]	Treasury Stock [Member]
Beginning Balances (shares) at Dec. 31, 2016	[1]			16,303,499
Beginning Balances at Dec. 31, 2016		$ 446,086	[1]	$ 7,935	$ 250,967	$ 192,062	$ (4,878)	$ 2,416	$ (2,416)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		58,124				58,124
Other comprehensive income (loss)		606					606
Dividend declared		(21,045)				(21,045)
Retirement savings plan and dividend reinvestment plan (shares)				10,771
Dividend reinvestment plan		735		$ 5	730
Stock issuance, shares				0
Stock issuance		(10)		$ 0	(10)
Share-based compensation (shares)	[2],[3]			30,172
Share-based compensation and tax benefit	[2],[3]	1,798		$ 15	1,783
Treasury stock activities		0						979	(979)
Ending Balances (shares) at Dec. 31, 2017	[1],[4]			16,344,442
Ending Balances at Dec. 31, 2017		$ 486,294	[4]	$ 7,955	253,470	229,141	(4,272)	3,395	(3,395)
Preferred Stock, Shares Authorized		2,000,000
Preferred Stock, Par or Stated Value Per Share		$ 0.01
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		$ 26,855				26,855
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect		(907)				907	(907)
Other comprehensive income (loss)		(1,694)					(1,694)
Dividend declared		(5,381)				(5,381)
Stock Issued During Period, Shares, Dividend Reinvestment Plan				0
Stock Issued During Period, Value, Dividend Reinvestment Plan		(1)		$ 0	(1)
Share-based compensation (shares)	[2],[3]			19,350
Share-based compensation and tax benefit	[2],[3]	666		$ 9	657
Treasury stock activities		0						178	(178)
Ending Balances (shares) at Mar. 31, 2018	[1],[4]			16,363,792
Ending Balances at Mar. 31, 2018		$ 505,241	[4]	$ 7,964	$ 254,126	250,024	$ (6,873)	$ 3,573	$ (3,573)
Preferred Stock, Shares Authorized		2,000,000
Preferred Stock, Par or Stated Value Per Share		$ 0.01
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ (1,498)				$ (1,498)

[1] 2,000,000 shares of preferred stock at $0.01 par value have been authorized. None has been issued or is outstanding; accordingly, no information has been included in the statements of stockholders’ equity.

[2] Includes amounts for shares issued for directors’ compensation.

[3] The shares issued under the SICP are net of shares withheld for employee taxes. For the three months ended March 31, 2018, and for the year ended December 31, 2017, we withheld 10,436 and 10,269 shares, respectively, for taxes.

[4] Includes 93,422 and 90,961 shares at March 31, 2018 and December 31, 2017, respectively, held in a Rabbi Trust related to our Deferred Compensation Plan.