Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net of tax
Balance	$ 2,731,341	$ 2,701,897	$ 2,744,104	$ 2,640,258
Net income	64,279	65,101	208,713	124,015
Other comprehensive income (loss)	(31,761)	(1,477)	(93,067)	83,003
Balance	2,711,734	2,733,934	2,711,734	2,733,934
Accumulated other comprehensive Income (Loss)
Net of tax
Balance	(29,702)	52,731	31,604	(31,749)
Other comprehensive income (loss)	(31,761)	(1,477)	(93,067)	83,003
Balance	(61,463)	51,254	(61,463)	51,254
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	2,035	80,909	63,341	(3,667)
Other comprehensive income (loss)	(31,761)	(1,477)	(93,067)	83,099
Balance	$ (29,726)	79,432	(29,726)	79,432
Reclassification of net losses (gains) to net income, Pre-tax Amount		$ 24	$ 102	$ (102)