Accumulated Other Comprehensive Income (Loss) - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net of tax
Balance, beginning	$ 2,640,258	$ 2,524,839
Net income	38,865	69,924
Other comprehensive income (loss)	35,878	53,441
Balance, ending	2,664,685	2,613,202
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	(31,749)	(132,195)
Other comprehensive income (loss)	35,878	53,441
Balance, ending	4,129	(78,754)
Pension and other benefits, net change in pension and other benefits
Net of tax
Balance, beginning	(28,082)	(28,379)
Other comprehensive income (loss)	(96)
Balance, ending	(28,178)	(28,379)
Investment securities:
Net of tax
Balance, beginning	(3,667)	(103,816)
Other comprehensive income (loss)	35,974	53,441
Balance, ending	32,307	(50,375)
Reclassification of net losses (gains) to net income	$ (85)	$ 2,613