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Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Net of Tax        
Balance at beginning of period $ 2,694,545 $ 2,550,312 $ 2,617,486 $ 2,486,066
Other comprehensive income (loss) 18,859 66,474 75,845 77,552
Balance at end of period 2,733,921 2,648,034 2,733,921 2,648,034
Accumulated Other Comprehensive Income (Loss)        
Net of Tax        
Balance at beginning of period (407,008) (519,132) (463,994) (530,210)
Other comprehensive income (loss) 18,859 66,474 75,845 77,552
Balance at end of period (388,149) (452,658) (388,149) (452,658)
Pensions and Other Benefits        
Net of Tax        
Balance at beginning of period (1,879) (5,373) (1,879) (5,373)
Balance at end of period (1,879) (5,373) (1,879) (5,373)
Investment securities        
Net of Tax        
Other comprehensive income (loss) 19,315 66,101 76,010 76,452
Available-for-Sale Investment Securities        
Net of Tax        
Balance at beginning of period (151,234) (228,076) (193,529) (222,423)
Other comprehensive income (loss) 10,908 55,893 53,203 50,240
Balance at end of period (140,326) (172,183) (140,326) (172,183)
Held-to-Maturity Investment Securities        
Net of Tax        
Balance at beginning of period (254,101) (285,607) (268,501) (301,611)
Other comprehensive income (loss) 8,407 10,208 22,807 26,212
Balance at end of period (245,694) (275,399) (245,694) (275,399)
Cash Flow Derivative Hedges        
Net of Tax        
Balance at beginning of period 206 (76) (85) (803)
Other comprehensive income (loss) (456) 373 (165) 1,100
Balance at end of period $ (250) $ 297 $ (250) $ 297