Loans Receivable and Allowance for Loan Losses - Summary of credit risk ratings by loan segment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment
Total loans	$ 1,594,413	$ 1,543,016
Commercial Real Estate
Financing Receivable, Recorded Investment
Total loans	1,042,689	987,242
Construction
Financing Receivable, Recorded Investment
Total loans	100,147	101,636
Commercial business
Financing Receivable, Recorded Investment
Total loans	262,625	259,995
Residential Real Estate
Financing Receivable, Recorded Investment
Total loans	188,546	193,524
Consumer
Financing Receivable, Recorded Investment
Total loans	406	619
Commercial Credit Quality Indicators
Financing Receivable, Recorded Investment
Total loans	1,405,461	1,348,873
Commercial Credit Quality Indicators | Pass
Financing Receivable, Recorded Investment
Total loans	1,357,246	1,315,108
Commercial Credit Quality Indicators | Special Mention
Financing Receivable, Recorded Investment
Total loans	18,366	13,390
Commercial Credit Quality Indicators | Substandard
Financing Receivable, Recorded Investment
Total loans	29,619	20,266
Commercial Credit Quality Indicators | Doubtful
Financing Receivable, Recorded Investment
Total loans	230	109
Commercial Credit Quality Indicators | Commercial Real Estate
Financing Receivable, Recorded Investment
Total loans	1,042,689	987,242
Commercial Credit Quality Indicators | Commercial Real Estate | Pass
Financing Receivable, Recorded Investment
Total loans	1,015,217	960,902
Commercial Credit Quality Indicators | Commercial Real Estate | Special Mention
Financing Receivable, Recorded Investment
Total loans	3,537	9,371
Commercial Credit Quality Indicators | Commercial Real Estate | Substandard
Financing Receivable, Recorded Investment
Total loans	23,809	16,969
Commercial Credit Quality Indicators | Commercial Real Estate | Doubtful
Financing Receivable, Recorded Investment
Total loans	126	0
Commercial Credit Quality Indicators | Construction
Financing Receivable, Recorded Investment
Total loans	100,147	101,636
Commercial Credit Quality Indicators | Construction | Pass
Financing Receivable, Recorded Investment
Total loans	89,662	101,636
Commercial Credit Quality Indicators | Construction | Special Mention
Financing Receivable, Recorded Investment
Total loans	10,485	0
Commercial Credit Quality Indicators | Construction | Substandard
Financing Receivable, Recorded Investment
Total loans	0	0
Commercial Credit Quality Indicators | Construction | Doubtful
Financing Receivable, Recorded Investment
Total loans	0	0
Commercial Credit Quality Indicators | Commercial business
Financing Receivable, Recorded Investment
Total loans	262,625	259,995
Commercial Credit Quality Indicators | Commercial business | Pass
Financing Receivable, Recorded Investment
Total loans	252,367	252,570
Commercial Credit Quality Indicators | Commercial business | Special Mention
Financing Receivable, Recorded Investment
Total loans	4,344	4,019
Commercial Credit Quality Indicators | Commercial business | Substandard
Financing Receivable, Recorded Investment
Total loans	5,810	3,297
Commercial Credit Quality Indicators | Commercial business | Doubtful
Financing Receivable, Recorded Investment
Total loans	104	109
Residential and Consumer Credit Quality Indicators
Financing Receivable, Recorded Investment
Total loans	188,952	194,143
Residential and Consumer Credit Quality Indicators | Pass
Financing Receivable, Recorded Investment
Total loans	181,302	189,536
Residential and Consumer Credit Quality Indicators | Special Mention
Financing Receivable, Recorded Investment
Total loans	328	0
Residential and Consumer Credit Quality Indicators | Substandard
Financing Receivable, Recorded Investment
Total loans	7,322	4,607
Residential and Consumer Credit Quality Indicators | Residential Real Estate
Financing Receivable, Recorded Investment
Total loans	188,546	193,524
Residential and Consumer Credit Quality Indicators | Residential Real Estate | Pass
Financing Receivable, Recorded Investment
Total loans	180,901	188,917
Residential and Consumer Credit Quality Indicators | Residential Real Estate | Special Mention
Financing Receivable, Recorded Investment
Total loans	328	0
Residential and Consumer Credit Quality Indicators | Residential Real Estate | Substandard
Financing Receivable, Recorded Investment
Total loans	7,317	4,607
Residential and Consumer Credit Quality Indicators | Consumer
Financing Receivable, Recorded Investment
Total loans	406	619
Residential and Consumer Credit Quality Indicators | Consumer | Pass
Financing Receivable, Recorded Investment
Total loans	401	619
Residential and Consumer Credit Quality Indicators | Consumer | Special Mention
Financing Receivable, Recorded Investment
Total loans	0	0
Residential and Consumer Credit Quality Indicators | Consumer | Substandard
Financing Receivable, Recorded Investment
Total loans	$ 5	$ 0