Quarterly Financial Data (Schedule Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Revenues	$ 416,505	$ 411,319	$ 405,532	$ 399,769	$ 406,941	$ 410,735	$ 410,704	$ 410,094	$ 1,633,125	$ 1,638,474	$ 1,526,996
Operating income	107,430	108,210	74,066	97,602	82,129	43,083	98,163	89,081	387,308	312,456	336,523
Depreciation, accretion, and amortization	(158,554)	(160,111)	(159,723)	(159,801)	(161,461)	(164,330)	(162,377)	(171,853)	(638,189)	(660,021)	(627,072)
Loss from extinguishment of debt, net	(18,189)	(34,512)			(783)				(52,701)	(783)	(26,204)
Net income (loss)	$ 5,256	$ (15,370)	$ 32,711	$ 53,641	$ 31,019	$ (155,946)	$ 28,305	$ (79,034)	$ 76,238	$ (175,656)	$ (24,295)
Net income (loss) per common share - basic	$ 0.04	$ (0.12)	$ 0.26	$ 0.43	$ 0.25	$ (1.23)	$ 0.22	$ (0.61)	$ 0.61	$ (1.37)	$ (0.19)
Net income (loss) per common share - diluted	$ 0.04	$ (0.12)	$ 0.26	$ 0.43	$ 0.24	$ (1.23)	$ 0.22	$ (0.61)	$ 0.61	$ (1.37)	$ (0.19)