Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net of Tax
Balance at beginning of period	$ 2,648,852	$ 2,513,761	$ 2,617,486	$ 2,486,066
Other comprehensive (loss) income	26,761	4,648	56,986	11,078
Balance at end of period	2,694,545	2,550,312	2,694,545	2,550,312
Accumulated Other Comprehensive Income (Loss)
Net of Tax
Balance at beginning of period	(433,769)	(523,780)	(463,994)	(530,210)
Other comprehensive (loss) income	26,761	4,648	56,986	11,078
Balance at end of period	(407,008)	(519,132)	(407,008)	(519,132)
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 (loss) income	26,643	4,684	56,695	10,351
Available-for-Sale Investment Securities
Net of Tax
Balance at beginning of period	(170,558)	(224,940)	(193,529)	(222,423)
Other comprehensive (loss) income	19,324	(3,136)	42,295	(5,653)
Balance at end of period	(151,234)	(228,076)	(151,234)	(228,076)
Held-to-Maturity Investment Securities
Net of Tax
Balance at beginning of period	(261,420)	(293,427)	(268,501)	(301,611)
Other comprehensive (loss) income	7,319	7,820	14,400	16,004
Balance at end of period	(254,101)	(285,607)	(254,101)	(285,607)
Cash Flow Derivative Hedges
Net of Tax
Balance at beginning of period	88	(40)	(85)	(803)
Other comprehensive (loss) income	118	(36)	291	727
Balance at end of period	$ 206	$ (76)	$ 206	$ (76)