Commitments and Credit Risk (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Credit Risk
Commitments to fund loans pending closing	$ 221,300,000	$ 263,800,000
Undisbursed Commitment on construction loans	101,100,000	209,200,000
Outstanding line of credit	613,800,000	586,900,000
Rental expenses	764,000	696,000	$ 574,000
Due within one year	874,000
Due in one year to two years	762,000
Due in two years to three years	763,000
Due in three years to four years	409,000
Due in four years to five years	231,000
Thereafter	470,000
Total minimum lease payments	3,509,000
Commitments to extend credit
Commitments and Credit Risk
Financial instrument whose contract amount represents credit risk	437,585,000	307,944,000
Irrevocable standby letters of credit
Commitments and Credit Risk
Financial instrument whose contract amount represents credit risk	26,000,000	26,000,000
Commitment letters of credit
Commitments and Credit Risk
Financial instrument whose contract amount represents credit risk	$ 31,031,000	$ 32,669,000
Minimum
Commitments and Credit Risk
Lease period	4 years
Maximum
Commitments and Credit Risk
Lease period	8 years