Consolidated Statement of Shareholders' Equity - 9 months ended Sep. 30, 2017 - USD ($) $ in Thousands	Total	Common Shares	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Restricted Shares	Restricted Shares Common Shares	Restricted Shares Treasury Stock
Balance, December 31, 2016 at Dec. 31, 2016	$ 435,261	$ 344,404	$ 110,294	$ (1,554)	$ (17,883)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	29,470		29,470
Other comprehensive income, net of tax	1,605			1,605
Cash dividends declared	(11,299)		(11,299)
Stock Issued During Period, Value, Stock Options Exercised	0	(6)			6
Reissuance of treasury stock for deferred compensation plan for Boards of Directors						$ 0	$ (1,467)	$ 1,467
Reissuance of treasury stock for deferred compensation plan for Boards of Directors	500				500
Repurchase of treasury stock in connection with employee incentive plan and under compensation plan for Boards of Directors	(411)				(411)
Common shares issued under dividend reinvestment plan	385	385
Common shares issued under compensation plan for Boards of Directors	240	(72)			168
Common shares issued under employee stock purchase plan	(273)	81			(192)
Stock-based compensation expense	1,362	1,362
Balance, September 30, 2017 at Sep. 30, 2017	$ 457,386	$ 344,831	$ 128,465	$ 51	$ (15,961)