Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive loss	$ (7,389)	$ (5,663)	$ (9,817)	$ (16,082)
Net current period other comprehensive (loss) income	11,551	15,948	(34,383)	117,059
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	28,159		78,754
Net unrealized gains arising during period	18,940	21,611	(24,566)	133,141
Amounts reclassified from accumulated other comprehensive loss	(12,189)	(10,589)	(19,278)	(25,831)
Net current period other comprehensive (loss) income	6,751	11,022	(43,844)	107,310
Ending balance	34,910	113,102	34,910	113,102
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	3,791		4,157
Net unrealized gains arising during period	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	(604)	0	(970)	0
Net current period other comprehensive (loss) income	(604)	0	(970)	0
Ending balance	3,187	0	3,187	0
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(65,621)		(70,648)
Net unrealized gains arising during period	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	5,404	4,926	10,431	9,749
Net current period other comprehensive (loss) income	5,404	4,926	10,431	9,749
Ending balance	(60,217)	(122,766)	(60,217)	(122,766)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(33,671)		12,263
Net unrealized gains arising during period	18,940	21,611	(24,566)	133,141
Amounts reclassified from accumulated other comprehensive loss	(7,389)	(5,663)	(9,817)	(16,082)
Net current period other comprehensive (loss) income	11,551	15,948	(34,383)	117,059
Ending balance	$ (22,120)	(9,664)	$ (22,120)	(9,664)
Previously Reported | Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		102,080		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		0
Previously Reported | Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(127,692)		(132,515)
Previously Reported | Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (25,612)		$ (126,723)