Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive loss	$ (1,340)	$ (11,622)	$ (11,157)	$ (27,704)
Net current period other comprehensive (loss) income	(10,017)	(4,092)	(44,400)	112,967
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	34,910		78,754
Net unrealized losses arising during period	(8,677)	7,530	(33,243)	140,671
Amounts reclassified from accumulated other comprehensive loss	(6,224)	(16,497)	(25,502)	(42,328)
Net current period other comprehensive (loss) income	(14,901)	(8,967)	(58,745)	98,343
Ending balance	20,009	104,135	20,009	104,135
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	3,187		4,157
Net unrealized losses arising during period	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	(331)	0	(1,301)	0
Net current period other comprehensive (loss) income	(331)	0	(1,301)	0
Ending balance	2,856	0	2,856	0
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(60,217)		(70,648)
Net unrealized losses arising during period	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	5,215	4,875	15,646	14,624
Net current period other comprehensive (loss) income	5,215	4,875	15,646	14,624
Ending balance	(55,002)	(117,891)	(55,002)	(117,891)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(22,120)		12,263
Net unrealized losses arising during period	(8,677)	7,530	(33,243)	140,671
Amounts reclassified from accumulated other comprehensive loss	(1,340)	(11,622)	(11,157)	(27,704)
Net current period other comprehensive (loss) income	(10,017)	(4,092)	(44,400)	112,967
Ending balance	$ (32,137)	(13,756)	$ (32,137)	(13,756)
Previously Reported | Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		113,102		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		(122,766)		(132,515)
Previously Reported | Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (9,664)		$ (126,723)