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Investment Securities - Available-for-Sale Securities Amortized Cost and Fair Value (Details) - USD ($) | Sep. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | $ 952,278,000 | $ 373,038,000
Available-for-sale, gross unrealized gains | 14,351,000 | 19,393,000
Available-for-sale, gross unrealized losses | (3,596,000) | (80,000)
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 963,033,000 | 392,351,000
Accrued interest | 2,900,000 | 1,900,000
MBS pass-through securities issued by FHLMC and FNMA |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 207,557,000 | 50,686,000
Available-for-sale, gross unrealized gains | 1,925,000 | 2,530,000
Available-for-sale, gross unrealized losses | (243,000) | 0
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 209,239,000 | 53,216,000
CMOs issued by FHLMC |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 363,708,000 | 143,267,000
Available-for-sale, gross unrealized gains | 5,045,000 | 7,925,000
Available-for-sale, gross unrealized losses | (1,065,000) | (1,000)
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 367,688,000 | 151,191,000
CMOs issued by FNMA |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 55,178,000 | 16,450,000
Available-for-sale, gross unrealized gains | 244,000 | 580,000
Available-for-sale, gross unrealized losses | (211,000) | 0
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 55,211,000 | 17,030,000
CMOs issued by GNMA |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 61,411,000 | 6,863,000
Available-for-sale, gross unrealized gains | 232,000 | 351,000
Available-for-sale, gross unrealized losses | (249,000) | 0
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 61,394,000 | 7,214,000
SBA-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 33,422,000 | 30,941,000
Available-for-sale, gross unrealized gains | 1,323,000 | 1,976,000
Available-for-sale, gross unrealized losses | (136,000) | (55,000)
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 34,609,000 | 32,862,000
Debentures of government- sponsored agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 71,328,000 | 19,944,000
Available-for-sale, gross unrealized gains | 63,000 | 266,000
Available-for-sale, gross unrealized losses | (1,177,000) | (24,000)
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 70,214,000 | 20,186,000
U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 11,881,000 |
Available-for-sale, gross unrealized gains | 0 |
Available-for-sale, gross unrealized losses | (104,000) |
Allowance for Credit Losses | 0 |
Available-for-sale securities | 11,777,000 |
Obligations of state and political subdivisions |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 131,865,000 | 104,887,000
Available-for-sale, gross unrealized gains | 5,519,000 | 5,765,000
Available-for-sale, gross unrealized losses | (169,000) | 0
Allowance for Credit Losses | 0 | 0
Available-for-sale securities | 137,215,000 | $ 110,652,000
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 14,014,000 |
Available-for-sale, gross unrealized gains | 0 |
Available-for-sale, gross unrealized losses | (237,000) |
Allowance for Credit Losses | 0 |
Available-for-sale securities | 13,777,000 |
Asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale, amortized cost | 1,914,000 |
Available-for-sale, gross unrealized gains | 0 |
Available-for-sale, gross unrealized losses | (5,000) |
Allowance for Credit Losses | 0 |
Available-for-sale securities | $ 1,909,000 |

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