Parent Company Financial Information - Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 30, 2011	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities:
Net income		$ 15,249	$ 16,301	$ 15,039
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sale of securities		(812)	(836)	(867)
Net change in other assets		(379)	(1,390)	1,778
Net change in other liabilities		284	867	(118)
Net cash provided by operating activities		25,751	16,117	19,262
Cash Flows from Investing Activities:
Purchases of securities available for sale		(84,931)	(168,069)	(120,040)
Proceeds from sales of securities available for sale		55,903	13,019	15,425
Cash paid in bank acquisition		0	0	(5,935)
Net cash used in investing activities		(147,856)	(175,366)	(17,364)
Cash Flows from Financing Activities:
Common stock dividends paid		(8,384)	(8,266)	(8,068)
Repurchase of common stock		0	(1,292)	(3,506)
Proceeds from exercise of stock options		113	142	789
Proceeds from issuance of common stock	$ 619
Net cash provided by financing activities		121,375	117,119	26,136
Net Increase (Decrease) in Cash and Cash Equivalents		(730)	(42,130)	28,034
Cash and Cash Equivalents at Beginning of Period		53,207	95,337	67,303
Cash and Cash Equivalents at End of Period		52,477	53,207	95,337
Parent Company
Cash Flows from Operating Activities:
Net income		15,249	16,301	15,039
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sale of securities		(221)	0	0
Equity in (undistributed) distributions of subsidiary		(9,942)	(1,320)	(4,749)
Net change in other assets		83	(57)	257
Net change in other liabilities		(82)	163	106
Net cash provided by operating activities		5,087	15,087	10,653
Cash Flows from Investing Activities:
Purchases of securities available for sale		(373)	(6,588)	0
Proceeds from sales of securities available for sale		500	0	0
Investment in banking subsidiary		0	0	563
Cash paid in bank acquisition		0	0	(5,935)
Net cash used in investing activities		127	(6,588)	(5,372)
Cash Flows from Financing Activities:
Common stock dividends paid		(8,384)	(8,266)	(8,068)
Repurchase of common stock		0	(1,292)	(3,506)
Proceeds from exercise of stock options		113	142	789
Proceeds from issuance of common stock		0	0	95
Net cash provided by financing activities		(8,271)	(9,416)	(10,690)
Net Increase (Decrease) in Cash and Cash Equivalents		(3,057)	(917)	(5,409)
Cash and Cash Equivalents at Beginning of Period		4,654	5,571	10,980
Cash and Cash Equivalents at End of Period		$ 1,597	$ 4,654	$ 5,571