Accumulated Other Comprehensive Income (Loss) (Schedule Of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), before Tax		$ (299,122)		$ (193,833)
Deferred tax expense (benefit)		(68,799)		(71,539)
Accumulated other comprehensive income (loss), net of tax		(230,323)	$ (110,941)	(122,294)	$ (135,192)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(2,569)	(1,574)
Amortization of actuarial losses and prior service cost		(3,317)	(2,448)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax		3,337	2,500
Unrealized Gains on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss), before Tax		(157,460)		(48,834)
Deferred tax expense (benefit)		(36,217)		(17,889)
Accumulated other comprehensive income (loss), net of tax	[1]	(121,243)	(23,183)	(30,945)	(45,875)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	0	(15)
Funded Status of Defined Benefit Plan
Accumulated Other Comprehensive Income (Loss), before Tax		(141,662)		(144,999)
Deferred tax expense (benefit)		(32,582)		(53,650)
Accumulated other comprehensive income (loss), net of tax	[1]	(109,080)	(87,758)	$ (91,349)	$ (89,317)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	$ (2,569)	$ (1,559)

[1] All amounts are net of tax.