Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net of tax
Balance, beginning	$ 2,664,685	$ 2,613,202	$ 2,640,258	$ 2,524,839
Net income	20,049	72,433	58,914	142,357
Other comprehensive income (loss)	48,602	46,770	84,480	100,211
Balance, ending	2,701,897	2,659,441	2,701,897	2,659,441
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	4,129	(78,754)	(31,749)	(132,195)
Other comprehensive income (loss)	48,602	46,770	84,480	100,211
Balance, ending	52,731	(31,984)	52,731	(31,984)
Pension and other benefits, net change in pension and other benefits
Net of tax
Balance, beginning	(28,178)	(28,379)	(28,082)	(28,379)
Other comprehensive income (loss)		(594)	(96)	(594)
Balance, ending	(28,178)	(28,973)	(28,178)	(28,973)
Investment securities:
Net of tax
Balance, beginning	32,307	(50,375)	(3,667)	(103,816)
Other comprehensive income (loss)	48,602	47,364	84,576	100,805
Balance, ending	80,909	(3,011)	80,909	(3,011)
Reclassification of net losses (gains) to net income	$ 211	$ (21)	$ 126	$ 2,592