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Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
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Investment securities held to maturity: |  |
Amortized Cost | $ 2,450,885 | $ 2,581,469
Gross Unrealized Gains | 15,380 | 0
Gross Unrealized Losses | 0 | (56,944)
Fair Value | 2,466,265 | 2,524,525
Investment securities available for sale: |  |
Amortized Cost | 7,862,915 | 8,079,918
Gross Unrealized Gains | 84,097 | 9,025
Gross Unrealized Losses | (371,544) | (537,925)
Fair Value | 7,575,468 | 7,551,018
Accrued interest receivable on investment securities held to maturity | 5,400 | 5,700
Discount included in amortized cost of investment securities HTM | (594,500) | (649,700)
Accrued interest receivable on investment securities available for sale | $ 29,400 | $ 29,500
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] | Other assets | Other assets
U.S. Treasury securities |  |
Investment securities available for sale: |  |
Amortized Cost | $ 1,200,313 | $ 1,214,363
Gross Unrealized Gains | 20,841 | 3,203
Gross Unrealized Losses | 0 | (4,824)
Fair Value | 1,221,154 | 1,212,742
U.S. Government agency securities |  |
Investment securities available for sale: |  |
Amortized Cost | 29,995 | 29,993
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (138) | (830)
Fair Value | 29,857 | 29,163
Mortgage-backed securities issued by U.S. Government agencies |  |
Investment securities available for sale: |  |
Amortized Cost | 1,504,894 | 1,583,331
Gross Unrealized Gains | 13,884 | 848
Gross Unrealized Losses | (89,072) | (121,389)
Fair Value | 1,429,706 | 1,462,790
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  |
Investment securities held to maturity: |  |
Amortized Cost | 2,450,885 | 2,581,469
Gross Unrealized Gains | 15,380 | 0
Gross Unrealized Losses | 0 | (56,944)
Fair Value | 2,466,265 | 2,524,525
Investment securities available for sale: |  |
Amortized Cost | 2,177,658 | 2,294,700
Gross Unrealized Gains | 5,172 | 250
Gross Unrealized Losses | (176,679) | (260,915)
Fair Value | 2,006,151 | 2,034,035
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  |
Investment securities available for sale: |  |
Amortized Cost | 603,527 | 657,453
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (84,328) | (107,252)
Fair Value | 519,199 | 550,201
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  |
Investment securities available for sale: |  |
Amortized Cost | 2,337,343 | 2,290,968
Gross Unrealized Gains | 44,194 | 4,724
Gross Unrealized Losses | (21,327) | (42,576)
Fair Value | 2,360,210 | 2,253,116
Corporate debt securities and other debt securities |  |
Investment securities available for sale: |  |
Amortized Cost | 9,185 | 9,110
Gross Unrealized Gains | 6 | 0
Gross Unrealized Losses | 0 | (139)
Fair Value | $ 9,191 | $ 8,971

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