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Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Sep. 30, 2024
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value $ 214,626 $ 456,673 $ 138,456
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss (1,378) (8,507) (410)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value 3,736,212 3,967,472 4,198,372
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss (412,552) (529,712) (421,788)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total 3,950,838 4,424,145 4,336,828
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss (413,930) (538,219) (422,198)
U.S. Treasury Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value 1,000 0 0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss 0 0 0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value 99,476 271,088 290,535
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss (897) (4,595) (5,671)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total 100,476 271,088 290,535
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss (897) (4,595) (5,671)
Obligations of State and Political Subdivisions [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value 21,278 56,777 6,899
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss (854) (1,224) (43)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value 1,344,598 1,361,628 1,395,597
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss (133,118) (137,618) (115,473)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total 1,365,876 1,418,405 1,402,496
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss (133,972) (138,842) (115,516)
Residential Mortgage-Backed Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value 125,497 198,385 124,439
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss (450) (4,990) (335)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value 2,042,826 2,048,727 2,205,425
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss (271,575) (373,756) (289,695)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total 2,168,323 2,247,112 2,329,864
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss (272,025) (378,746) (290,030)
Commercial Mortgage-Backed Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value 66,851 196,640 7,118
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss (74) (2,249) (32)
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value 166,330 190,189 209,632
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss (3,344) (7,409) (5,763)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total 233,181 386,829 216,750
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss (3,418) (9,658) (5,795)
Corporate Bonds and Other [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value 0 4,871 0
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss 0 (44) 0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value 82,982 95,840 97,183
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss (3,618) (6,334) (5,186)
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair value,Total 82,982 100,711 97,183
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss $ (3,618) $ (6,378) $ (5,186)