Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pre-tax Amount
Other comprehensive income	$ 49,079	$ 73,136
Income Tax Benefit (Expense)
Other comprehensive income	(13,201)	(19,695)
Net of tax
Balance, beginning	2,640,258	2,524,839
Other comprehensive income	35,878	53,441
Balance, ending	2,664,685	2,613,202
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Balance	(43,450)	(180,915)
Balance	5,629	(107,779)
Income Tax Benefit (Expense)
Balance	11,701	48,720
Balance	(1,500)	29,025
Net of tax
Balance, beginning	(31,749)	(132,195)
Other comprehensive income	35,878	53,441
Balance, ending	4,129	(78,754)
Pension and other benefits, change in Company tax rate
Income Tax Benefit (Expense)
Other comprehensive income	(96)
Net of tax
Other comprehensive income	(96)
Pension and other benefits, net change in pension and other benefits
Income Tax Benefit (Expense)
Other comprehensive income	(96)
Net of tax
Balance, beginning	(28,082)	(28,379)
Other comprehensive income	(96)
Balance, ending	(28,178)	(28,379)
Investment securities:
Pre-tax Amount
Unrealized net gains arising during the year	49,164	70,523
Reclassification of net losses (gains) to net income	(85)	2,613
Other comprehensive income	49,079	73,136
Income Tax Benefit (Expense)
Unrealized net gains arising during the year	(13,128)	(18,991)
Reclassification of net losses (gains) to net income	(23)	(704)
Other comprehensive income	(13,105)	(19,695)
Net of tax
Balance, beginning	(3,667)	(103,816)
Unrealized net gains arising during the year	36,036	51,532
Reclassification of net losses (gains) to net income	(62)	1,909
Other comprehensive income	35,974	53,441
Balance, ending	$ 32,307	$ (50,375)