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Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Schedule of Investments [Line Items] |  |
Amortized Cost | $ 3,024,001 |
Gross Unrealized Gains | 75,149 |
Gross Unrealized Losses | (14,813) |
Estimated Fair Value | 3,084,337 | $ 3,062,143
Amortized Cost, Held to Maturity | 916,423 | 278,281
Gross Unrealized Gains, Held to Maturity | 13,803 |
Gross Unrealized Losses, Held to Maturity | (20,070) |
HTM securities | 910,156 |
US Government Debt Securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 153,493 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (283) |
Estimated Fair Value | 153,210 |
US Government Agencies Debt Securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 62,137 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (71) |
Estimated Fair Value | 62,066 |
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 1,111,024 | 891,327
Gross Unrealized Gains | 43,139 | 61,286
Gross Unrealized Losses | (3,769) | 0
Estimated Fair Value | 1,150,394 | 952,613
Corporate Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 354,727 | 348,391
Gross Unrealized Gains | 14,663 | 19,445
Gross Unrealized Losses | (52) | (691)
Estimated Fair Value | 369,338 | 367,145
Collateralized Mortgage Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 246,726 | 491,321
Gross Unrealized Gains | 5,639 | 12,560
Gross Unrealized Losses | (330) | (115)
Estimated Fair Value | 252,035 | 503,766
Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 191,623 | 373,779
Gross Unrealized Gains | 1,669 | 4,246
Gross Unrealized Losses | (3,169) | (27)
Estimated Fair Value | 190,123 | 377,998
Amortized Cost, Held to Maturity | 410,189 | 278,281
Gross Unrealized Gains, Held to Maturity | 13,803 | 18,576
Gross Unrealized Losses, Held to Maturity | (6,514) | 0
HTM securities | 417,478 | 296,857
Commercial Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 827,921 | 741,172
Gross Unrealized Gains | 10,039 | 22,384
Gross Unrealized Losses | (5,922) | (1,141)
Estimated Fair Value | 832,038 | 762,415
Amortized Cost, Held to Maturity | 506,234 |
Gross Unrealized Gains, Held to Maturity | 0 |
Gross Unrealized Losses, Held to Maturity | (13,556) |
HTM securities | 492,678 |
Auction Rate Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 76,350 | 101,510
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1,217) | (3,304)
Estimated Fair Value | $ 75,133 | 98,206
Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost |  | 2,947,500
Gross Unrealized Gains |  | 119,921
Gross Unrealized Losses |  | (5,278)
Estimated Fair Value |  | $ 3,062,143

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