Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Net of tax
Balance	$ 2,329,012	$ 2,285,149	$ 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 income (loss)	(1,057)	(53,955)	26,518	(449,764)
Balance	2,359,738	2,252,611	2,359,738	2,252,611	2,269,005
Accumulated Other Comprehensive Income (Loss)
Net of tax
Balance	(611,679)	(517,502)	(639,254)	(121,693)	(121,693)
Other comprehensive income (loss)	(1,057)	(53,955)	26,518	(449,764)
Balance	(612,736)	(571,457)	(612,736)	(571,457)	(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 income (loss)	(705)	(52,371)	26,239	(446,922)
Available-for-sale Investment securities
Net of tax
Balance	(273,816)	(491,854)	(292,175)	(97,303)	(97,303)
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		338,816		338,816
Other comprehensive income (loss)	(10,284)	(66,983)	8,075	(461,534)
Balance	(284,100)	(220,021)	(284,100)	(220,021)	(292,175)
Held-to-Maturity Investment Securities
Net of tax
Balance	(328,361)		(336,946)
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity		(338,816)		(338,816)
Other comprehensive income (loss)	9,579	14,612	18,164	14,612
Balance	(318,782)	(324,204)	(318,782)	(324,204)	(336,946)
Cash Flow Derivative Hedges
Net of tax
Balance	(4,071)	(1,258)	(4,702)
Other comprehensive income (loss)	(352)	(1,584)	279	(2,842)
Balance	$ (4,423)	$ (2,842)	$ (4,423)	$ (2,842)	$ (4,702)