Consolidated Statements of Shareholders' Equity $ in Thousands	USD ($)
Shareholders' equity at beginning of year: at Dec. 31, 2014	$ 399,496
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,283
Other comprehensive income (loss)	(14,372)
Cash dividends paid to shareholders	(15,003)
Issuance of common stock	1,094
Repurchase of common shares	0
Shareholders' equity at end of year: at Dec. 31, 2015	394,498
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	28,945
Other comprehensive income (loss)	(4,638)
Cash dividends paid to shareholders	(15,803)
Issuance of common stock	1,343
Repurchase of common shares	0
Shareholders' equity at end of year: at Dec. 31, 2016	404,345
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	18,323
Other comprehensive income (loss)	13,171
Cash dividends paid to shareholders	(16,302)
Issuance of common stock	1,154
Repurchase of common shares	(1,880)
Shareholders' equity at end of year: at Dec. 31, 2017	$ 418,811