Other Comprehensive Income (Loss) (Component Of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	$ 2,562
Other comprehensive loss before reclassifications	(23,410)	$ 1,022
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	(3,175)	0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	(2,024)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2,024)	0
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(23,012)	1,022
Other comprehensive income (loss)	(21,386)	1,022
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Reclassification Adjustments, Net of Tax	(23,410)	1,022
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax		0
Changes in pension plan assets and benefit obligations, Net-of-tax amount	(26,701)	(14,740)	$ 23,526	$ 14,740
Unrealized gains on available-for-sale securities, Net-of-tax amount	(25,940)	(1,983)	(2,928)	(3,005)
Total accumulated other comprehensive income (loss), Net-of-tax amount	(52,641)	(16,723)	(26,454)	$ (17,745)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax		$ 0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			(3,175)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			(631)
Accumulated Other Comprehensive Income (loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			(3,806)
Accounting Standards Update 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss)	$ (26,187)
Accounting Standards Update 2016-01 [Member] | Accumulated Other Comprehensive Income (loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ (995)