Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Net of tax
Balance	$ 2,285,149	$ 2,683,630	$ 2,656,912	$ 2,744,104
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity	338,800		338,800
Other comprehensive loss	(53,955)	13,733	(449,764)	(61,306)
Balance	2,252,611	2,731,341	2,252,611	2,731,341
Reclassification of net losses (gains) to net income, Pre-tax Amount				102
Accumulated Other Comprehensive Income (Loss)
Net of tax
Balance	(517,502)	(43,435)	(121,693)	31,604
Other comprehensive loss	(53,955)	13,733	(449,764)	(61,306)
Balance	(571,457)	(29,702)	(571,457)	(29,702)
Pensions and Other Benefits
Net of tax
Balance	(24,390)	(31,737)	(24,390)	(31,737)
Balance	(24,390)	(31,737)	(24,390)	(31,737)
Investment Securities.
Net of tax
Other comprehensive loss	(52,371)	13,733	(446,922)	(61,306)
Reclassification of net losses (gains) to net income, Pre-tax Amount	(19,929)	102	(19,929)
Available-for-sale Investment securities
Net of tax
Balance	(491,854)	(11,698)	(97,303)	63,341
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity	338,816		338,816
Other comprehensive loss	(66,983)	13,733	(461,534)	(61,306)
Balance	(220,021)	$ 2,035	(220,021)	$ 2,035
Held-to-Maturity Investment Securities
Net of tax
Unrealized net losses related to the transfer of securities from available-for-sale to held-to-maturity	(338,816)		(338,816)
Other comprehensive loss	14,612		14,612
Balance	(324,204)		(324,204)
Cash Flow Derivative Hedges
Net of tax
Balance	(1,258)
Other comprehensive loss	(1,584)		(2,842)
Balance	(2,842)		(2,842)
Reclassification of net losses (gains) to net income, Pre-tax Amount	$ 638		$ 638