Accumulated Other Comprehensive Loss (Schedule Of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), before Tax		$ 15,926	$ (164,576)
Deferred tax expense (benefit)		3,663	(37,853)
Accumulated other comprehensive income (loss), net of tax		12,263	(126,723)
Income taxes	$ (13,900)	(126,159)	(134,677)	$ (103,297)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		27,276	(18,292)
Unrealized gains on securities available for sale
Accumulated Other Comprehensive Income (Loss), before Tax		102,278	7,522
Deferred tax expense (benefit)		23,524	1,730
Accumulated other comprehensive income (loss), net of tax		78,754	5,792
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		46,395	5,479
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss), before Tax		(91,751)	(172,098)
Deferred tax expense (benefit)		(21,103)	(39,583)
Accumulated other comprehensive income (loss), net of tax		(70,648)	(132,515)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(19,500)	(8,456)
Reclassification adjustment for losses included in income before income taxes			(10,924)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax		(25,324)	(10,981)
Unrealized gains on securities available for sale transferred from (to) held to maturity
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Before Tax		495	(19,889)
Accumulated Other Comprehensive Income (Loss), before Tax		5,399	0
Deferred tax expense (benefit)		1,242	0
Accumulated other comprehensive income (loss), net of tax		4,157	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		381	(15,315)
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Tax		(114)	4,574
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Net Of Tax		$ 381	$ (15,315)