Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net of tax
Balance	$ 2,744,104	$ 2,640,258
Net income	57,693	38,865
Other comprehensive income (loss)	(75,039)	35,878
Balance	2,683,630	2,664,685
Accumulated other comprehensive Income (loss)
Net of tax
Balance	31,604	(31,749)
Other comprehensive income (loss)	(75,039)	35,878
Balance	(43,435)	4,129
Pension and other benefits:
Net of tax
Balance	(31,737)	(28,082)
Other comprehensive income (loss)		(96)
Balance	(31,737)	(28,178)
Investment securities:
Net of tax
Balance	63,341	(3,667)
Other comprehensive income (loss)	(75,039)	35,974
Balance	$ (11,698)	32,307
Reclassification of net losses (gains) to net income, Pre-tax Amount		$ 85