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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Asset retirement obligation    
Asset retirement obligation - Balance at beginning of year $ 239,032 [1] $ 275,238
Additional liabilities accrued 1,661 4,907
Revisions in estimated cash outflows (3,669) (992)
Disposition of assets (9,684) (46,242)
Accretion expense 15,735 17,506
Transferred to Liabilities held for sale   (11,385)
Asset retirement obligation - Balance at end of year [1] 243,075 239,032
Other current liabilities    
Asset retirement obligation    
Asset retirement obligation - Balance at beginning of year $ 9,100  
Asset retirement obligation - Balance at end of year   $ 9,100
[1] The total amount of asset retirement obligations related to the Divestiture Transaction and Airadigm Transaction included in Other current liabilities was $9.1 million as of December 31, 2014.