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Securities - Amortized Cost and Fair Value of Debt Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 850,672 | $ 737,005
Gross Unrealized Gains | 21,392 | 33,478
Gross Unrealized Losses | (3,070) | (90)
Fair Value | 868,994 | 770,393
Interest receivable | 3,200 | 3,100
U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 85,222 | 9,980
Gross Unrealized Gains | 326 | 302
Gross Unrealized Losses | (248) | 0
Fair Value | 85,300 | 10,282
Obligations of U.S. government corporations and agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 68,635 | 78,755
Gross Unrealized Gains | 2,570 | 4,149
Gross Unrealized Losses | 0 | 0
Fair Value | 71,205 | 82,904
Collateralized mortgage obligations of U.S. government corporations and agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 206,325 | 202,975
Gross Unrealized Gains | 3,858 | 6,410
Gross Unrealized Losses | (1,198) | (89)
Fair Value | 208,985 | 209,296
Residential mortgage-backed securities of U.S. government corporations and agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 60,855 | 66,960
Gross Unrealized Gains | 553 | 818
Gross Unrealized Losses | (966) | 0
Fair Value | 60,442 | 67,778
Commercial mortgage-backed securities of U.S. government corporations and agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 347,825 | 258,875
Gross Unrealized Gains | 9,339 | 14,806
Gross Unrealized Losses | (658) | 0
Fair Value | 356,506 | 273,681
Corporate obligations |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 500 | 2,021
Gross Unrealized Gains | 0 | 5
Gross Unrealized Losses | 0 | (1)
Fair Value | 500 | 2,025
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 81,310 | 117,439
Gross Unrealized Gains | 4,746 | 6,988
Gross Unrealized Losses |  | 0
Fair Value | $ 86,056 | $ 124,427

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
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