Loans, net and allowance for loan losses - Additional Information (Details)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2016 USD ($) loan	Sep. 30, 2016 USD ($) loan	Sep. 30, 2015 USD ($) loan	Dec. 31, 2015 USD ($)
Net deferred loan costs	$ 544,000	$ 544,000		$ 690,000
Loans receivable, related parties, considered as nonaccrual, past due or restructured or potential credit risk		2,666,000	$ 2,962,000	2,861,000
Number of defaults on loans restructured			0
Loans modified as troubled debt restructuring		75,000	542,000
Total impaired loans	$ 12,758,000	$ 12,758,000	$ 15,918,000	12,635,000
Number of loans modified as troubled debt restructuring \| loan	0	1	8
Real estate Residential
Number of defaults on loans restructured		$ 2
Loans modified as troubled debt restructuring		208,000
Total impaired loans	$ 3,264,000	$ 3,264,000	$ 4,883,000	$ 4,966,000

Deferred Loan Costs Net

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- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

+ Details

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524

+ Details

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- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

+ Details

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- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

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- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524

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