Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	$ (2,428)	$ (10,419)
Net current period other comprehensive (loss) income	(45,934)	101,111
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	78,754
Net unrealized losses arising during period	(43,506)	111,530
Amounts reclassified from accumulated other comprehensive income	(7,089)	(15,242)
Net current period other comprehensive (loss) income	(50,595)	96,288
Ending balance	28,159	102,080
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	4,157
Net unrealized losses arising during period	0	0
Amounts reclassified from accumulated other comprehensive income	(366)	0
Net current period other comprehensive (loss) income	(366)	0
Ending balance	3,791	0
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(70,648)
Net unrealized losses arising during period	0	0
Amounts reclassified from accumulated other comprehensive income	5,027	4,823
Net current period other comprehensive (loss) income	5,027	4,823
Ending balance	(65,621)	(127,692)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	12,263
Net unrealized losses arising during period	(43,506)	111,530
Amounts reclassified from accumulated other comprehensive income	(2,428)	(10,419)
Net current period other comprehensive (loss) income	(45,934)	101,111
Ending balance	$ (33,671)	(25,612)
Previously Reported | Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		5,792
Previously Reported | Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		0
Previously Reported | Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(132,515)
Previously Reported | Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (126,723)