Loans - Changes in the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in the Allowance for Loan Losses
Balance, beginning of period				$ 19,089				$ 18,926	$ 19,089	$ 18,926	$ 18,379
Charge-offs									(2,239)	(2,007)	(538)
Recoveries									2,709	933	1,053
Net (charge-offs) recoveries									470	(1,074)	515
Provision for loan losses	$ (291)	$ 115	$ (46)	321	$ 240	$ 245	$ 351	401	99	1,237	32
Balance, end of period	19,658				19,089				19,658	19,089	18,926
Commercial
Changes in the Allowance for Loan Losses
Balance, beginning of period				10,656				10,748	10,656	10,748	11,187
Charge-offs									(2,239)	(1,966)	(527)
Recoveries									1,585	365	877
Net (charge-offs) recoveries									(654)	(1,601)	350
Provision for loan losses									606	1,509	(789)
Balance, end of period	10,608				10,656				10,608	10,656	10,748
Real estate
Changes in the Allowance for Loan Losses
Balance, beginning of period				8,327				8,076	8,327	8,076	7,070
Charge-offs											(2)
Recoveries									1,124	568	146
Net (charge-offs) recoveries									1,124	568	144
Provision for loan losses									(501)	(317)	862
Balance, end of period	8,950				8,327				8,950	8,327	8,076
Consumer
Changes in the Allowance for Loan Losses
Balance, beginning of period				$ 106				$ 102	106	102	122
Charge-offs										(41)	(9)
Recoveries											30
Net (charge-offs) recoveries										(41)	21
Provision for loan losses									(6)	45	(41)
Balance, end of period	$ 100				$ 106				$ 100	$ 106	$ 102