Investment Securities - Cost basis, unrealized gains and losses, and fair value of debt securities held to maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Held-to-maturity securities:
Amortized Cost	$ 283,533	$ 274,407
Gross Unrealized Gains	5,215	1,777
Gross Unrealized Losses	844	7,381
Fair value	287,904	268,803
State and political subdivisions
Held-to-maturity securities:
Amortized Cost	45,479	68,551
Gross Unrealized Gains	1,574	952
Gross Unrealized Losses	9	2,191
Fair value	47,044	67,312
Residential mortgage-backed securities, Agency
Held-to-maturity securities:
Amortized Cost	153,967	176,488
Gross Unrealized Gains	2,014	652
Gross Unrealized Losses	694	5,094
Fair value	155,287	172,046
Commercial mortgage-backed, Agency
Held-to-maturity securities:
Amortized Cost	84,087	29,368
Gross Unrealized Gains	1,627	173
Gross Unrealized Losses	141	96
Fair value	$ 85,573	$ 29,445

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- Definition

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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- Definition

Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

+ References

No definition available.

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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