Securities - Amortized cost, gross unrealized gains and losses, and fair value of debt securities held-to-maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt securities held to maturity:
Amortized Cost	$ 398,373	$ 283,533
Gross Unrealized Gains	15,599	5,215
Gross Unrealized Losses	152	844
Debt securities held to maturity, fair value	413,820	287,904
U.S. Government agencies
Debt securities held to maturity:
Amortized Cost	10,616
Gross Unrealized Gains	146
Gross Unrealized Losses	0
Debt securities held to maturity, fair value	10,762
State and political subdivisions
Debt securities held to maturity:
Amortized Cost	163,180	45,479
Gross Unrealized Gains	6,200	1,574
Gross Unrealized Losses	28	9
Debt securities held to maturity, fair value	169,352	47,044
Residential mortgage-backed securities, Agency
Debt securities held to maturity:
Amortized Cost	125,011	153,967
Gross Unrealized Gains	5,149	2,014
Gross Unrealized Losses	0	694
Debt securities held to maturity, fair value	130,160	155,287
Commercial mortgage-backed, Agency
Debt securities held to maturity:
Amortized Cost	99,566	84,087
Gross Unrealized Gains	4,104	1,627
Gross Unrealized Losses	124	141
Debt securities held to maturity, fair value	$ 103,546	$ 85,573

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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

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