Investment Securities - Cost basis, unrealized gains and losses, and fair value of debt securities held to maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Held-to-maturity securities:
Amortized Cost	$ 274,407	$ 321,094
Gross Unrealized Gains	1,777	3,785
Gross Unrealized Losses	7,381	3,603
Fair value	268,803	321,276
State and political subdivisions
Held-to-maturity securities:
Amortized Cost	68,551	71,959
Gross Unrealized Gains	952	1,574
Gross Unrealized Losses	2,191	178
Fair value	67,312	73,355
Residential mortgage-backed securities
Held-to-maturity securities:
Amortized Cost	176,488	208,805
Gross Unrealized Gains	652	1,586
Gross Unrealized Losses	5,094	3,289
Fair value	172,046	207,102
Commercial mortgage-backed securities
Held-to-maturity securities:
Amortized Cost	29,368	40,330
Gross Unrealized Gains	173	625
Gross Unrealized Losses	96	136
Fair value	$ 29,445	$ 40,819

Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

+ References

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- Definition

Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

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- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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