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Securities - Gross Unrealized Losses and Fair Value of Investment Securities Held to Maturity (Detail) $ in Thousands | Mar. 31, 2026 USD ($) Security | Dec. 31, 2025 USD ($) Security
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Schedule Of Held To Maturity Securities [Line Items] |  |
Less than 12 months, Count | Security | 281 | 159
Less than 12 months, Fair Value | $ 1,159,990 | $ 850,159
Less than 12 months,Unrealized Losses | $ (57,097) | $ (42,077)
12 months or more, Count | Security | 1,615 | 1,616
12 months or more, Fair Value | $ 3,445,636 | $ 3,479,693
12 months or more, Unrealized Losses | $ (486,583) | $ (458,760)
Total Count | Security | 1,896 | 1,775
Total Fair Value | $ 4,605,626 | $ 4,329,852
Total Unrealized Losses | $ (543,680) | $ (500,837)
U.S. Treasury [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Less than 12 months, Count | Security | 7 | 3
Less than 12 months, Fair Value | $ 37,958 | $ 15,913
Less than 12 months,Unrealized Losses | $ (298) | $ (37)
Total Count | Security | 7 | 3
Total Fair Value | $ 37,958 | $ 15,913
Total Unrealized Losses | $ (298) | $ (37)
Mortgage-backed [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Less than 12 months, Count | Security | 14 | 10
Less than 12 months, Fair Value | $ 172,121 | $ 147,066
Less than 12 months,Unrealized Losses | $ (1,682) | $ (918)
12 months or more, Count | Security | 262 | 262
12 months or more, Fair Value | $ 1,951,706 | $ 1,998,984
12 months or more, Unrealized Losses | $ (306,012) | $ (304,122)
Total Count | Security | 276 | 272
Total Fair Value | $ 2,123,827 | $ 2,146,050
Total Unrealized Losses | $ (307,694) | $ (305,040)
State and political subdivisions [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Less than 12 months, Count | Security | 260 | 146
Less than 12 months, Fair Value | $ 949,911 | $ 687,180
Less than 12 months,Unrealized Losses | $ (55,117) | $ (41,122)
12 months or more, Count | Security | 1,353 | 1,354
12 months or more, Fair Value | $ 1,493,930 | $ 1,480,709
12 months or more, Unrealized Losses | $ (180,571) | $ (154,638)
Total Count | Security | 1,613 | 1,500
Total Fair Value | $ 2,443,841 | $ 2,167,889
Total Unrealized Losses | $ (235,688) | $ (195,760)

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