Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Net of tax
Balance	$ 2,359,738	$ 2,252,611	$ 2,269,005	$ 2,656,912	$ 2,656,912
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity					(372,400)
Other comprehensive (loss) income	(35,995)	(87,299)	(9,477)	(537,063)
Balance	2,351,009	2,200,651	2,351,009	2,200,651	2,269,005
Accumulated other comprehensive loss
Net of tax
Balance	(612,736)	(571,457)	(639,254)	(121,693)	(121,693)
Other comprehensive (loss) income	(35,995)	(87,299)	(9,477)	(537,063)
Balance	(648,731)	(658,756)	(648,731)	(658,756)	(639,254)
Pensions and Other Benefits
Net of tax
Balance	(5,431)	(24,390)	(5,431)	(24,390)	(24,390)
Balance	(5,431)	(24,390)	(5,431)	(24,390)	(5,431)
Investment securities
Net of tax
Other comprehensive (loss) income	(36,985)	(84,164)	(10,746)	(531,086)
Available-for-sale Investment securities
Net of tax
Balance	(284,100)	(220,021)	(292,175)	(97,303)	(97,303)
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		33,603		372,419
Other comprehensive (loss) income	(45,369)	(95,323)	(37,294)	(556,857)
Balance	(329,469)	(281,741)	(329,469)	(281,741)	(292,175)
Held-to-Maturity Investment Securities
Net of tax
Balance	(318,782)	(324,204)	(336,946)
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		(33,603)		(372,419)
Other comprehensive (loss) income	8,384	11,159	26,548	25,771
Balance	(310,398)	(346,648)	(310,398)	(346,648)	(336,946)
Cash Flow Derivative Hedges
Net of tax
Balance	(4,423)	(2,842)	(4,702)
Other comprehensive (loss) income	990	(3,135)	1,269	(5,977)
Balance	$ (3,433)	$ (5,977)	$ (3,433)	$ (5,977)	$ (4,702)