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Investment Securities - Amortized Cost of Investment Securities Held to Maturity (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | $ 2,071,183 | $ 1,227,668
Aaa |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 151,669 |
Aa1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 146,992 |
Aa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 161,730 |
Aa3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 124,369 |
A1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 88,277 |
A2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 23,103 |
A3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,066 |
Baa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 266 |
Non-rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,373,711 |
State and municipal |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 996,247 | $ 619,927
State and municipal | Aaa |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 90,991 |
State and municipal | Aa1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 146,992 |
State and municipal | Aa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 161,730 |
State and municipal | Aa3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 124,369 |
State and municipal | A1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 88,277 |
State and municipal | A2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 23,103 |
State and municipal | A3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,066 |
State and municipal | Baa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 266 |
State and municipal | Non-rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 359,453 |
Other |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 1,074,936 |
Other | Aaa |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 60,678 |
Other | Aa1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | Aa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | Aa3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | A1 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | A2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | A3 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | Baa2 |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 0 |
Other | Non-rated |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | $ 1,014,258 |

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