Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Net of Tax
Balance at beginning of period	$ 2,617,486	$ 2,486,066
Other comprehensive (loss) income	30,225	6,430
Balance at end of period	2,648,852	2,513,761
Accumulated Other Comprehensive Income (Loss)
Net of Tax
Balance at beginning of period	(463,994)	(530,210)
Other comprehensive (loss) income	30,225	6,430
Balance at end of period	(433,769)	(523,780)
Pensions and Other Benefits
Net of Tax
Balance at beginning of period	(1,879)	(5,373)
Balance at end of period	(1,879)	(5,373)
Investment securities
Net of Tax
Other comprehensive (loss) income	30,052	5,667
Available-for-Sale Investment Securities
Net of Tax
Balance at beginning of period	(193,529)	(222,423)
Other comprehensive (loss) income	22,971	(2,517)
Balance at end of period	(170,558)	(224,940)
Held-to-Maturity Investment Securities
Net of Tax
Balance at beginning of period	(268,501)	(301,611)
Other comprehensive (loss) income	7,081	8,184
Balance at end of period	(261,420)	(293,427)
Cash Flow Derivative Hedges
Net of Tax
Balance at beginning of period	(85)	(803)
Other comprehensive (loss) income	173	763
Balance at end of period	$ 88	$ (40)