Consolidated Statement of Shareholders' Equity - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Total	Common Shares	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock	Restricted Shares	Restricted Shares Common Shares	Restricted Shares Treasury Stock
Balance, December 31, 2017 at Dec. 31, 2017	$ 458,592	$ 345,412	$ 134,362	$ (5,215)	$ (15,967)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified out of retained earnings per ASU 2014-09 | Accounting Standards Update 2014-09 [Member]			(3,055)
Net income	11,741		11,741
Amounts reclassified out of accumulated other comprehensive loss per ASU 2016-01 (a)	5,020
Other comprehensive income, net of tax	(5,827)			(10,847)
Cash dividends declared	(4,771)		(4,771)
Stock Issued During Period, Value, Stock Options Exercised	0	(3)			3
Reissuance of treasury stock for deferred compensation plan for Boards of Directors						$ 0	$ (2,281)	$ 2,281
Reissuance of treasury stock for deferred compensation plan for Boards of Directors	2				2
Repurchase of treasury stock in connection with employee incentive plan and under compensation plan for Boards of Directors	(1,056)				(1,056)
Common shares issued under dividend reinvestment plan	161	161
Common shares issued under compensation plan for Boards of Directors	0	84			84
Stock-based compensation expense	1,028	1,028
Balance, March 31, 2018 at Mar. 31, 2018	$ 456,815	$ 344,233	$ 143,297	$ (16,062)	$ (14,653)