Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net of tax
Balance	$ 2,269,005	$ 2,656,912	$ 2,744,104
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		(372,400)
Other comprehensive income (loss)	109,044	(517,561)	(153,297)
Balance	2,486,066	2,269,005	2,656,912
Incremental non-credit-related impairment loss recognized in earnings on securities	0	0	0
Accumulated other comprehensive loss
Net of tax
Balance	(639,254)	(121,693)	31,604
Other comprehensive income (loss)	109,044	(517,561)	(153,297)
Balance	(530,210)	(639,254)	(121,693)
Pensions and Other Benefits
Net of tax
Balance	(5,431)	(24,390)	(31,737)
Other comprehensive income (loss)	58	18,959	7,347
Balance	(5,373)	(5,431)	(24,390)
Investment securities
Net of tax
Other comprehensive income (loss)	105,087	(531,818)
Available-for-sale Investment securities
Net of tax
Balance	(292,175)	(97,303)	63,341
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		372,419
Other comprehensive income (loss)	69,752	(567,291)	(160,644)
Balance	(222,423)	(292,175)	$ (97,303)
Held-to-Maturity Investment Securities
Net of tax
Balance	(336,946)
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		(372,419)
Other comprehensive income (loss)	35,335	35,473
Balance	(301,611)	(336,946)
Cash Flow Derivative Hedges.
Net of tax
Balance	(4,702)
Other comprehensive income (loss)	3,899	(4,702)
Balance	$ (803)	$ (4,702)