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Investment Securities - Amortized Cost of Investment Securities Held to Maturity (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | $ 1,295,534 | $ 1,227,668
Aaa |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 109,365 |
Aa1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 84,954 |
Aa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 107,436 |
Aa3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 72,969 |
A1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 65,578 |
A2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 19,074 |
A3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 1,065 |
Baa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 528 |
Non-rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 834,565 |
State and municipal |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 624,553 | $ 619,927
State and municipal | Aaa |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 48,689 |
State and municipal | Aa1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 84,954 |
State and municipal | Aa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 107,436 |
State and municipal | Aa3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 72,969 |
State and municipal | A1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 65,578 |
State and municipal | A2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 19,074 |
State and municipal | A3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 1,065 |
State and municipal | Baa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 528 |
State and municipal | Non-rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 224,260 |
Other |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 670,981 |
Other | Aaa |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 60,676 |
Other | Aa1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | Aa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | Aa3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | A1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | A2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | A3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | Baa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | 0 |
Other | Non-rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to maturity securities | $ 610,305 |

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