Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pre-tax Amount
Other comprehensive income (loss), Pre-tax Amount	$ (102,336)	$ 49,079
Income Tax Benefit (Expense)
Other comprehensive income (loss), Income Tax Benefit (Expense)	27,297	(13,201)
Net of tax
Balance	2,744,104	2,640,258
Other comprehensive (loss) income	(75,039)	35,878
Balance	2,683,630	2,664,685
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Accumulated other comprehensive loss, beginning balance, Pre-tax Amount	43,098	(43,450)
Accumulated other comprehensive loss, ending balance, Pre-tax Amount	(59,238)	5,629
Income Tax Benefit (Expense)
Accumulated other comprehensive loss, beginning balance, Income Tax Benefit (Expense)	(11,494)	11,701
Accumulated other comprehensive loss, ending balance, Income Tax Benefit (Expense)	15,803	(1,500)
Net of tax
Balance	31,604	(31,749)
Other comprehensive (loss) income	(75,039)	35,878
Balance	(43,435)	4,129
Pension and other benefits:
Income Tax Benefit (Expense)
Other comprehensive income (loss), Income Tax Benefit (Expense)		(96)
Net of tax
Balance	(31,737)	(28,082)
Other comprehensive (loss) income		(96)
Balance	(31,737)	(28,178)
Pension and other benefits, change in Company tax rate
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)		(96)
Net of tax
Unrealized net gains arising during the period, Net of tax		(96)
Investment securities:
Pre-tax Amount
Unrealized net gains (losses) arising during the period, Pre-tax Amount	(102,336)	49,164
Reclassification of net losses (gains) to net income, Pre-tax Amount		(85)
Other comprehensive income (loss), Pre-tax Amount	(102,336)	49,079
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	27,297	(13,128)
Reclassification of net losses (gains) to net income, Income Tax Benefit (Expense)		23
Other comprehensive income (loss), Income Tax Benefit (Expense)	27,297	(13,105)
Net of tax
Balance	63,341	(3,667)
Unrealized net gains arising during the period, Net of tax	(75,039)	36,036
Reclassification of net gains to net income, Net of tax		(62)
Other comprehensive (loss) income	(75,039)	35,974
Balance	$ (11,698)	$ 32,307