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Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail)
$ in Thousands
Sep. 30, 2025
USD ($)
Security
Dec. 31, 2024
USD ($)
Security
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 267 527
Less than 12 months, Fair Value $ 1,508,371 $ 2,170,778
Less than 12 months, Unrealized Losses $ (21,645) $ (31,313)
12 months or more, Count | Security 2,215 2,721
12 months or more, Fair Value $ 4,038,950 $ 4,554,200
12 months or more, Unrealized Losses $ (445,690) $ (606,852)
Total Count | Security 2,482 3,248
Total Fair Value $ 5,547,321 $ 6,724,978
Total Unrealized Losses $ (467,335) $ (638,165)
U.S. Treasury [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 7 93
Less than 12 months, Fair Value $ 79,522 $ 635,739
Less than 12 months, Unrealized Losses $ (168) $ (6,319)
12 months or more, Count | Security 7 9
12 months or more, Fair Value $ 75,611 $ 142,518
12 months or more, Unrealized Losses $ (582) $ (1,753)
Total Count | Security 14 102
Total Fair Value $ 155,133 $ 778,257
Total Unrealized Losses $ (750) $ (8,072)
U.S. Agencies [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 1 4
Less than 12 months, Fair Value $ 8,583 $ 20,858
Less than 12 months, Unrealized Losses $ (167) $ (46)
12 months or more, Count | Security   5
12 months or more, Fair Value   $ 56,712
12 months or more, Unrealized Losses   $ (279)
Total Count | Security 1 9
Total Fair Value $ 8,583 $ 77,570
Total Unrealized Losses $ (167) $ (325)
Mortgage-backed [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 99 159
Less than 12 months, Fair Value $ 697,377 $ 1,293,953
Less than 12 months, Unrealized Losses $ (3,896) $ (22,468)
12 months or more, Count | Security 829 834
12 months or more, Fair Value $ 2,951,837 $ 3,055,882
12 months or more, Unrealized Losses $ (369,853) $ (502,489)
Total Count | Security 928 993
Total Fair Value $ 3,649,214 $ 4,349,835
Total Unrealized Losses $ (373,749) $ (524,957)
State and political subdivisions [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 147 264
Less than 12 months, Fair Value $ 610,974 $ 173,006
Less than 12 months, Unrealized Losses $ (17,306) $ (2,392)
12 months or more, Count | Security 1,215 1,629
12 months or more, Fair Value $ 800,722 $ 953,458
12 months or more, Unrealized Losses $ (68,985) $ (88,652)
Total Count | Security 1,362 1,893
Total Fair Value $ 1,411,696 $ 1,126,464
Total Unrealized Losses $ (86,291) $ (91,044)
Corporates [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 2  
Less than 12 months, Fair Value $ 2,244  
Less than 12 months, Unrealized Losses $ (6)  
12 months or more, Count | Security 161 239
12 months or more, Fair Value $ 194,933 $ 315,109
12 months or more, Unrealized Losses $ (6,221) $ (13,629)
Total Count | Security 163 239
Total Fair Value $ 197,177 $ 315,109
Total Unrealized Losses $ (6,227) $ (13,629)
Collateralized loan obligations [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 11 7
Less than 12 months, Fair Value $ 109,671 $ 47,222
Less than 12 months, Unrealized Losses $ (102) $ (88)
12 months or more, Count | Security 3 5
12 months or more, Fair Value $ 15,847 $ 30,521
12 months or more, Unrealized Losses $ (49) $ (50)
Total Count | Security 14 12
Total Fair Value $ 125,518 $ 77,743
Total Unrealized Losses $ (151) $ (138)