Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive loss	$ (5,663)	$ 2,281	$ (16,082)	$ 8,993
Net current period other comprehensive income	15,948	26,129	117,059	54,456
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	102,080		5,792
Net unrealized gains arising during period	21,611	23,848	133,141	45,463
Amounts reclassified from accumulated other comprehensive loss	(10,589)	(4,404)	(25,831)	(4,404)
Net current period other comprehensive income	11,022	19,444	107,310	41,059
Ending balance	113,102	2,554	113,102	2,554
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	0		0
Net unrealized gains arising during period	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	0	4,579	0	9,170
Net current period other comprehensive income	0	4,579	0	9,170
Ending balance	0	(61,979)	0	(61,979)
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(127,692)		(132,515)
Net unrealized gains arising during period	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	4,926	2,106	9,749	4,227
Net current period other comprehensive income	4,926	2,106	9,749	4,227
Ending balance	(122,766)	(121,306)	(122,766)	(121,306)
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(25,612)		(126,723)
Net unrealized gains arising during period	21,611	23,848	133,141	45,463
Amounts reclassified from accumulated other comprehensive loss	(5,663)	2,281	(16,082)	8,993
Net current period other comprehensive income	15,948	26,129	117,059	54,456
Ending balance	$ (9,664)	(180,731)	$ (9,664)	(180,731)
Previously Reported | Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(16,890)		(38,505)
Previously Reported | Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(66,558)		(71,149)
Previously Reported | Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(123,412)		(125,533)
Previously Reported | Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (206,860)		$ (235,187)