Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net of tax
Balance, beginning	$ 2,701,897	$ 2,659,441	$ 2,640,258	$ 2,524,839
Net income	65,101	74,199	124,015	216,556
Other comprehensive income (loss)	(1,477)	13,210	83,003	113,421
Balance, ending	2,733,934	2,654,558	2,733,934	2,654,558
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	52,731	(31,984)	(31,749)	(132,195)
Other comprehensive income (loss)	(1,477)	13,210	83,003	113,421
Balance, ending	51,254	(18,774)	51,254	(18,774)
Pension and other benefits, net change in pension and other benefits
Net of tax
Balance, beginning	(28,178)	(28,973)	(28,082)	(28,379)
Other comprehensive income (loss)			(96)	(594)
Balance, ending	(28,178)	(28,973)	(28,178)	(28,973)
Investment securities:
Net of tax
Balance, beginning	80,909	(3,011)	(3,667)	(103,816)
Other comprehensive income (loss)	(1,477)	13,210	83,099	114,015
Balance, ending	79,432	$ 10,199	79,432	10,199
Reclassification of net losses (gains) to net income	$ (24)		$ 102	$ 2,592