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Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | $ 5,784,754 | $ 5,313,209
Unrealized Gains | 197,714 | 93,542
Unrealized Losses | (11,385) | (21,660)
Basis Adjustments | (968) | 0
Fair Value | 5,970,115 | 5,385,091
U.S. Treasury |  |
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | 9,909 | 17,567
Unrealized Gains | 299 | 117
Unrealized Losses | 0 | (2)
Basis Adjustments | 0 | 0
Fair Value | 10,208 | 17,682
U.S. government-sponsored entities and agencies |  |
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | 841,133 | 596,595
Unrealized Gains | 5,744 | 1,027
Unrealized Losses | (3,921) | (4,638)
Basis Adjustments | (968) | 0
Fair Value | 841,988 | 592,984
Mortgage-backed securities |  |
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | 3,249,002 | 3,151,550
Unrealized Gains | 91,086 | 41,363
Unrealized Losses | (990) | (9,052)
Basis Adjustments | 0 | 0
Fair Value | 3,339,098 | 3,183,861
States and political subdivisions |  |
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | 1,405,868 | 1,232,497
Unrealized Gains | 86,325 | 44,193
Unrealized Losses | (31) | (1,047)
Basis Adjustments | 0 | 0
Fair Value | 1,492,162 | 1,275,643
Pooled trust preferred securities |  |
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | 13,763 | 13,811
Unrealized Gains | 0 | 0
Unrealized Losses | (5,850) | (5,589)
Basis Adjustments | 0 | 0
Fair Value | 7,913 | 8,222
Other securities |  |
Schedule Of Available For Sale And Held To Maturity Securities [Line Items] |  |
Amortized Cost | 265,079 | 301,189
Unrealized Gains | 14,260 | 6,842
Unrealized Losses | (593) | (1,332)
Basis Adjustments | 0 | 0
Fair Value | $ 278,746 | $ 306,699

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