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Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Investments [Line Items] |  |
Amortized Cost | $ 2,735,991 |
Gross Unrealized Gains | 77,413 |
Gross Unrealized Losses | (22,570) |
Estimated Fair Value | 2,790,834 | $ 2,497,537
Amortized Cost, Held to Maturity | 350,606 | 369,841
HTM securities | 372,273 |
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 825,863 |
Gross Unrealized Gains | 23,790 |
Gross Unrealized Losses | (1,569) |
Estimated Fair Value | 848,084 |
Amortized Cost |  | 638,125
Gross Unrealized Gains |  | 15,826
Gross Unrealized Losses |  | (1,024)
Estimated Fair Value |  | 652,927
Corporate Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 375,141 |
Gross Unrealized Gains | 5,378 |
Gross Unrealized Losses | (7,041) |
Estimated Fair Value | 373,478 |
Amortized Cost |  | 370,401
Gross Unrealized Gains |  | 8,490
Gross Unrealized Losses |  | (1,534)
Estimated Fair Value |  | 377,357
Collateralized Mortgage Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 638,458 |
Gross Unrealized Gains | 29,194 |
Gross Unrealized Losses | 0 |
Estimated Fair Value | 667,652 |
Amortized Cost |  | 682,307
Gross Unrealized Gains |  | 11,726
Gross Unrealized Losses |  | (315)
Estimated Fair Value |  | 693,718
Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 214,259 |
Gross Unrealized Gains | 6,370 |
Gross Unrealized Losses | 0 |
Estimated Fair Value | 220,629 |
Amortized Cost, Held to Maturity | 350,606 | 369,841
Gross Unrealized Gains, Held to Maturity | 21,667 | 13,864
Gross Unrealized Losses, Held to Maturity | 0 | 0
HTM securities | 372,273 | 383,705
Amortized Cost |  | 177,183
Gross Unrealized Gains |  | 1,078
Gross Unrealized Losses |  | (949)
Estimated Fair Value |  | 177,312
Commercial Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 574,860 |
Gross Unrealized Gains | 12,681 |
Gross Unrealized Losses | (216) |
Estimated Fair Value | 587,325 |
Amortized Cost |  | 489,603
Gross Unrealized Gains |  | 6,471
Gross Unrealized Losses |  | (1,777)
Estimated Fair Value |  | 494,297
Auction Rate Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost | 107,410 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (13,744) |
Estimated Fair Value | $ 93,666 |
Amortized Cost |  | 107,410
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | (5,484)
Estimated Fair Value |  | 101,926
Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Amortized Cost |  | 2,465,029
Gross Unrealized Gains |  | 43,591
Gross Unrealized Losses |  | (11,083)
Estimated Fair Value |  | $ 2,497,537

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