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Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Schedule of Investments [Line Items] |  |
Amortized Cost | $ 3,009,064 |
Gross Unrealized Gains | 94,744 |
Gross Unrealized Losses | (6,433) |
Estimated Fair Value | 3,097,375 | $ 3,062,143
Amortized Cost, Held to Maturity | 824,283 | 278,281
Gross Unrealized Gains, Held to Maturity | 15,470 |
Gross Unrealized Losses, Held to Maturity | (12,821) |
HTM securities | 826,932 |
US Government Debt Securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 153,885 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (340) |
Estimated Fair Value | 153,545 |
US Government Agencies Debt Securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 62,529 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (98) |
Estimated Fair Value | 62,431 |
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 1,002,146 | 891,327
Gross Unrealized Gains | 53,924 | 61,286
Gross Unrealized Losses | (608) | 0
Estimated Fair Value | 1,055,462 | 952,613
Corporate Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 354,593 | 348,391
Gross Unrealized Gains | 16,876 | 19,445
Gross Unrealized Losses | (88) | (691)
Estimated Fair Value | 371,381 | 367,145
Collateralized Mortgage Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 299,333 | 491,321
Gross Unrealized Gains | 7,965 | 12,560
Gross Unrealized Losses | (133) | (115)
Estimated Fair Value | 307,165 | 503,766
Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 201,654 | 373,779
Gross Unrealized Gains | 1,641 | 4,246
Gross Unrealized Losses | (1,748) | (27)
Estimated Fair Value | 201,547 | 377,998
Amortized Cost, Held to Maturity | 439,220 | 278,281
Gross Unrealized Gains, Held to Maturity | 15,470 | 18,576
Gross Unrealized Losses, Held to Maturity | (5,019) | 0
HTM securities | 449,671 | 296,857
Commercial Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 858,574 | 741,172
Gross Unrealized Gains | 14,338 | 22,384
Gross Unrealized Losses | (1,902) | (1,141)
Estimated Fair Value | 871,010 | 762,415
Amortized Cost, Held to Maturity | 385,063 |
Gross Unrealized Gains, Held to Maturity | 0 |
Gross Unrealized Losses, Held to Maturity | (7,802) |
HTM securities | 377,261 |
Auction Rate Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 76,350 | 101,510
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1,516) | (3,304)
Estimated Fair Value | $ 74,834 | 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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