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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
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)