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Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Investment securities held to maturity: |  |
Amortized Cost | $ 2,668,068 | $ 0
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (29,997) |
Fair Value | 2,638,071 |
Investment securities available for sale: |  |
Amortized Cost | 7,571,447 | 11,091,254
Gross Unrealized Gains | 7,172 | 31,540
Gross Unrealized Losses | (534,938) | (1,334,132)
Fair Value | 7,043,681 | 9,788,662
Accrued interest receivable on investment securities held to maturity | 5,900 |
Discount included in amortized cost of investment securities HTM | (687,900) |
Accrued interest receivable on investment securities available for sale | $ 27,700 | $ 26,600
Debt securities, available-for-sale, accrued interest, after allowance for credit loss, noncurrent, statement of financial position [extensible enumeration] | Other assets | Other assets
U.S. Treasury securities |  |
Investment securities available for sale: |  |
Amortized Cost | $ 1,177,006 | $ 588,082
Gross Unrealized Gains | 1,456 | 9,547
Gross Unrealized Losses | (4,167) | 0
Fair Value | 1,174,295 | 597,629
U.S. Government agency securities |  |
Investment securities available for sale: |  |
Amortized Cost | 29,993 | 29,993
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1,150) | (1,053)
Fair Value | 28,843 | 28,940
Mortgage-backed securities issued by U.S. Government agencies |  |
Investment securities available for sale: |  |
Amortized Cost | 1,518,224 | 1,021,612
Gross Unrealized Gains | 1,311 | 2,037
Gross Unrealized Losses | (119,043) | (97,985)
Fair Value | 1,400,492 | 925,664
Mortgage-backed securities issued by U.S. Government sponsored enterprises |  |
Investment securities held to maturity: |  |
Amortized Cost | 2,668,068 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (29,997) |
Fair Value | 2,638,071 |
Investment securities available for sale: |  |
Amortized Cost | 2,272,694 | 7,523,399
Gross Unrealized Gains | 287 | 1,192
Gross Unrealized Losses | (256,936) | (1,094,212)
Fair Value | 2,016,045 | 6,430,379
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises |  |
Investment securities available for sale: |  |
Amortized Cost | 659,706 | 692,487
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (111,709) | (104,892)
Fair Value | 547,997 | 587,595
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises |  |
Investment securities available for sale: |  |
Amortized Cost | 1,904,765 | 1,226,672
Gross Unrealized Gains | 4,118 | 18,764
Gross Unrealized Losses | (41,757) | (35,653)
Fair Value | 1,867,126 | 1,209,783
Corporate debt securities and other debt securities |  |
Investment securities available for sale: |  |
Amortized Cost | 9,059 | 9,009
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (176) | (337)
Fair Value | $ 8,883 | $ 8,672

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