Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net of tax
Balance	$ 2,683,630	$ 2,664,685	$ 2,744,104	$ 2,640,258
Net income	86,741	20,049	144,434	58,914
Other comprehensive income (loss)	13,733	48,602	(61,306)	84,480
Balance	2,731,341	2,701,897	2,731,341	2,701,897
Accumulated other comprehensive Income (Loss)
Net of tax
Balance	(43,435)	4,129	31,604	(31,749)
Other comprehensive income (loss)	13,733	48,602	(61,306)	84,480
Balance	(29,702)	52,731	(29,702)	52,731
Pension and other benefits:
Net of tax
Balance	(31,737)	(28,178)	(31,737)	(28,082)
Other comprehensive income (loss)				(96)
Balance	(31,737)	(28,178)	(31,737)	(28,178)
Investment securities:
Net of tax
Balance	(11,698)	32,307	63,341	(3,667)
Other comprehensive income (loss)	13,733	48,602	(61,306)	84,576
Balance	2,035	80,909	2,035	80,909
Reclassification of net losses (gains) to net income, Pre-tax Amount	$ 102	$ (211)	$ 102	$ (126)