Loans, net and allowance for loan losses - Changes in Allowance for Loan Losses Account by Major Classification of Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 2016
Allowance for loan losses:
Beginning Balance	$ 15,961	$ 12,975
Charge-offs	(311)	(123)
Recoveries	119	106
Provisions	1,200	1,200
Ending balance	16,969	14,158
Beginning Balance	15,961	12,975
Ending balance: individually evaluated for impairment			$ 1,424	$ 1,942
Charge-offs	(311)	(123)
Ending balance: collectively evaluated for impairment			15,545	14,019
Recoveries	119	106
Ending balance: loans acquired with deteriorated credit quality	15,961	12,975	16,969	15,961
Provisions	1,200	1,200
Loans receivable:
Total Loans			1,559,867	1,532,965
Ending balance: individually evaluated for impairment			9,726	11,242
Ending balance: collectively evaluated for impairment			1,548,393	1,519,386
Total Loans			1,559,867	1,532,965
Ending balance	16,969	14,158
Ending balance: individually evaluated for impairment			9,726	11,242
Ending balance: collectively evaluated for impairment			1,548,393	1,519,386
Loans Acquired with Deteriorated Credit Quality
Loans receivable:
Ending balance			1,748	2,337
Ending balance			1,748	2,337
Commercial
Allowance for loan losses:
Beginning Balance	3,799	3,042
Charge-offs		(3)
Recoveries	7	2
Provisions	323	281
Ending balance	4,129	3,322
Beginning Balance	3,799	3,042
Ending balance: individually evaluated for impairment			404	225
Charge-offs		(3)
Ending balance: collectively evaluated for impairment			3,725	3,574
Recoveries	7	2
Ending balance: loans acquired with deteriorated credit quality	3,799	3,042	4,129	3,799
Provisions	323	281
Loans receivable:
Total Loans			431,164	408,814
Ending balance: individually evaluated for impairment			2,311	1,724
Ending balance: collectively evaluated for impairment			428,476	406,127
Total Loans			431,164	408,814
Ending balance	4,129	3,322
Ending balance: individually evaluated for impairment			2,311	1,724
Ending balance: collectively evaluated for impairment			428,476	406,127
Commercial | Loans Acquired with Deteriorated Credit Quality
Loans receivable:
Ending balance			377	963
Ending balance			377	963
Real estate Commercial
Allowance for loan losses:
Beginning Balance	5,847	4,245
Charge-offs	(125)	(55)
Recoveries	33	16
Provisions	536	410
Ending balance	6,291	4,616
Beginning Balance	5,847	4,245
Ending balance: individually evaluated for impairment			513	1,197
Charge-offs	(125)	(55)
Ending balance: collectively evaluated for impairment			5,778	4,650
Recoveries	33	16
Ending balance: loans acquired with deteriorated credit quality	5,847	4,245	6,291	5,847
Provisions	536	410
Loans receivable:
Total Loans			703,194	700,144
Ending balance: individually evaluated for impairment			3,849	5,820
Ending balance: collectively evaluated for impairment			698,008	692,987
Total Loans			703,194	700,144
Ending balance	6,291	4,616
Ending balance: individually evaluated for impairment			3,849	5,820
Ending balance: collectively evaluated for impairment			698,008	692,987
Real estate Commercial | Loans Acquired with Deteriorated Credit Quality
Loans receivable:
Ending balance			1,337	1,337
Ending balance			1,337	1,337
Real estate Residential
Allowance for loan losses:
Beginning Balance	4,707	4,082
Charge-offs	(15)
Recoveries	22	25
Provisions	264	252
Ending balance	4,978	4,359
Beginning Balance	4,707	4,082
Ending balance: individually evaluated for impairment			484	520
Charge-offs	(15)
Ending balance: collectively evaluated for impairment			4,494	4,187
Recoveries	22	25
Ending balance: loans acquired with deteriorated credit quality	4,707	4,082	4,978	4,707
Provisions	264	252
Loans receivable:
Total Loans			286,389	289,781
Ending balance: individually evaluated for impairment			3,359	3,543
Ending balance: collectively evaluated for impairment			282,996	286,201
Total Loans			286,389	289,781
Ending balance	4,978	4,359
Ending balance: individually evaluated for impairment			3,359	3,543
Ending balance: collectively evaluated for impairment			282,996	286,201
Real estate Residential | Loans Acquired with Deteriorated Credit Quality
Loans receivable:
Ending balance			34	37
Ending balance			34	37
Consumer
Allowance for loan losses:
Beginning Balance	1,608	1,583
Charge-offs	(171)	(65)
Recoveries	57	63
Provisions	77	65
Ending balance	1,571	1,646
Beginning Balance	1,608	1,583
Ending balance: individually evaluated for impairment			23
Charge-offs	(171)	(65)
Ending balance: collectively evaluated for impairment			1,548	1,608
Recoveries	57	63
Ending balance: loans acquired with deteriorated credit quality	1,608	1,583	1,571	1,608
Provisions	77	65
Loans receivable:
Total Loans			139,120	134,226
Ending balance: individually evaluated for impairment			207	155
Ending balance: collectively evaluated for impairment			138,913	134,071
Total Loans			139,120	134,226
Ending balance	$ 1,571	1,646
Ending balance: individually evaluated for impairment			207	155
Ending balance: collectively evaluated for impairment			$ 138,913	$ 134,071
Unallocated
Allowance for loan losses:
Beginning Balance		23
Provisions		192
Ending balance		215
Beginning Balance		23
Ending balance: loans acquired with deteriorated credit quality		23
Provisions		192
Loans receivable:
Ending balance		$ 215