Investment securities - Additional Information (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Available-for-sale and Held-to-maturity securities
Carrying value of securities pledged \| $	$ 165,205	$ 144,750
Number of investment securities held	74	163
US Government Agency and Sponsored Enterprises
Available-for-sale and Held-to-maturity securities
Number of investment securities held	29
Number of securities in continuous unrealized loss positions 12 months or longer	10
State and Municipals, Tax-exempt
Available-for-sale and Held-to-maturity securities
Number of investment securities held	11	107
Number of securities in continuous unrealized loss positions 12 months or longer	4
State and Municipals, Taxable
Available-for-sale and Held-to-maturity securities
Number of investment securities held	1	2
Credit Concentration Risk \| Total Stockholders' Equity \| US Government Agency and Sponsored Enterprises \| Minimum
Available-for-sale and Held-to-maturity securities
Maximum percentage of stockholders' equity exceeded for securities of any individual issuer	10.00%	10.00%
U.S. Government-sponsored enterprises state and municipals
Available-for-sale and Held-to-maturity securities
Number of investment securities held		22
Number of securities in continuous unrealized loss positions 12 months or longer		9
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Available-for-sale and Held-to-maturity securities
Number of investment securities held	31	30
Number of securities in continuous unrealized loss positions 12 months or longer	14
U.S. Treasuries
Available-for-sale and Held-to-maturity securities
Number of investment securities held	2	2

The number of securities in continuous unrealized loss position.

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- Definition

Number of securities in continuous unrealized loss position for more than 12 months.

+ References

No definition available.

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- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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