Summary Of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (1,716,896)	$ (1,545,010)	$ (1,638,943)
Net other comprehensive (income) loss	(2,991)	1,517	(4,792)
Ending balance	(1,809,842)	(1,716,896)	(1,545,010)
Gain on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(964)	(557)
Change excluding amounts reclassified from accumulated other comprehensive loss	(1,228)	(407)
Amounts reclassified from accumulated other comprehensive loss	651
Net other comprehensive (income) loss	(577)	(407)
Ending balance	(1,541)	(964)	(557)
Gain on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6,403)	(8,160)
Change excluding amounts reclassified from accumulated other comprehensive loss	(1,063)	3,288
Amounts reclassified from accumulated other comprehensive loss	42	(1,531)
Net other comprehensive (income) loss	(1,021)	1,757
Ending balance	(7,424)	(6,403)	(8,160)
Defined Benefit Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	16,528	16,361
Change excluding amounts reclassified from accumulated other comprehensive loss	82	1,719
Amounts reclassified from accumulated other comprehensive loss	(1,475)	(1,552)
Net other comprehensive (income) loss	(1,393)	167
Ending balance	15,135	16,528	16,361
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	9,161	7,644	12,436
Change excluding amounts reclassified from accumulated other comprehensive loss	(2,209)	4,600
Amounts reclassified from accumulated other comprehensive loss	(782)	(3,083)
Net other comprehensive (income) loss	(2,991)	1,517	(4,792)
Ending balance	$ 6,170	$ 9,161	$ 7,644