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Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail)
$ in Thousands
Mar. 31, 2026
USD ($)
Security
Dec. 31, 2025
USD ($)
Security
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 796 264
Less than 12 months, Fair Value $ 3,537,971 $ 1,519,913
Less than 12 months, Unrealized Losses $ (36,868) $ (17,173)
12 months or more, Count | Security 1,727 2,096
12 months or more, Fair Value $ 3,651,505 $ 3,878,183
12 months or more, Unrealized Losses $ (408,394) $ (391,573)
Total Count | Security 2,523 2,360
Total Fair Value $ 7,189,476 $ 5,398,096
Total Unrealized Losses $ (445,262) $ (408,746)
U.S. Treasury [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 77 8
Less than 12 months, Fair Value $ 594,413 $ 72,013
Less than 12 months, Unrealized Losses $ (2,942) $ (88)
12 months or more, Count | Security 1 2
12 months or more, Fair Value $ 14,815 $ 30,234
12 months or more, Unrealized Losses $ (178) $ (353)
Total Count | Security 78 10
Total Fair Value $ 609,228 $ 102,247
Total Unrealized Losses $ (3,120) $ (441)
U.S. Agencies [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 1 1
Less than 12 months, Fair Value $ 7,569 $ 7,855
Less than 12 months, Unrealized Losses $ (66) $ (100)
Total Count | Security 1 1
Total Fair Value $ 7,569 $ 7,855
Total Unrealized Losses $ (66) $ (100)
Mortgage-backed [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 212 82
Less than 12 months, Fair Value $ 1,987,192 $ 757,160
Less than 12 months, Unrealized Losses $ (18,766) $ (5,682)
12 months or more, Count | Security 798 817
12 months or more, Fair Value $ 2,696,163 $ 2,871,729
12 months or more, Unrealized Losses $ (325,259) $ (325,469)
Total Count | Security 1,010 899
Total Fair Value $ 4,683,355 $ 3,628,889
Total Unrealized Losses $ (344,025) $ (331,151)
State and political subdivisions [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 474 152
Less than 12 months, Fair Value $ 631,510 $ 515,364
Less than 12 months, Unrealized Losses $ (14,306) $ (11,181)
12 months or more, Count | Security 837 1,142
12 months or more, Fair Value $ 806,914 $ 809,113
12 months or more, Unrealized Losses $ (79,697) $ (61,666)
Total Count | Security 1,311 1,294
Total Fair Value $ 1,438,424 $ 1,324,477
Total Unrealized Losses $ (94,003) $ (72,847)
Corporates [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security   1
Less than 12 months, Fair Value   $ 2,990
Less than 12 months, Unrealized Losses   $ (10)
12 months or more, Count | Security 89 134
12 months or more, Fair Value $ 120,650 $ 164,108
12 months or more, Unrealized Losses $ (3,217) $ (4,078)
Total Count | Security 89 135
Total Fair Value $ 120,650 $ 167,098
Total Unrealized Losses $ (3,217) $ (4,088)
Collateralized loan obligations [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 32 20
Less than 12 months, Fair Value $ 317,287 $ 164,531
Less than 12 months, Unrealized Losses $ (788) $ (112)
12 months or more, Count | Security 2 1
12 months or more, Fair Value $ 12,963 $ 2,999
12 months or more, Unrealized Losses $ (43) $ (7)
Total Count | Security 34 21
Total Fair Value $ 330,250 $ 167,530
Total Unrealized Losses $ (831) $ (119)