Accumulated Other Comprehensive Loss (Schedule Of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), before Tax		$ (193,833)	$ (214,481)
Long-term obligations		1,059,896	944,675		$ 924,905
Deferred tax expense (benefit)		(71,539)	(79,289)
Accumulated other comprehensive income (loss), net of tax		(122,294)	(135,192)		(64,440)
Income taxes		(219,946)	(125,585)		(122,028)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	(3,419)	12,351
Securities gains (losses)		4,293	26,673		10,817
Unrealized Gains on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss), before Tax		(48,834)	(72,707)
Deferred tax expense (benefit)		(17,889)	(26,832)
Accumulated other comprehensive income (loss), net of tax	[2]	(30,945)	(45,875)		(15,125)
Income taxes	[1]	(1,588)	(9,869)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1],[2]	2,705	16,804
Securities gains (losses)	[1]	4,293	26,673
Funded Status of Defined Benefit Plan
Accumulated Other Comprehensive Income (Loss), before Tax		(144,999)	(141,774)
Deferred tax expense (benefit)		(53,650)	(52,457)
Accumulated other comprehensive income (loss), net of tax	[2]	(91,349)	(89,317)		(48,423)
Income taxes	[1]	3,596	2,616
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1],[2]	(6,124)	(4,453)
Employee Benefits, prior service costs	[1]	(210)	(210)
Reclassification adjustment for losses included in income before income taxes	[1]	(9,510)	(6,859)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax		(9,720)	(7,069)	[1]
Unrealized Loss on Cash Flow Hedges
Accumulated other comprehensive income (loss), net of tax	[2]	0	0		$ (892)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[2]	$ 0	$ 0

[1] Amounts in parentheses indicate debits to profit/loss.

[2] All amounts are net of tax. Amounts in parentheses indicate debits.