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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 275,452 $ 323,026
Accretion (1) 104,659 35,740
New obligations 28,447 7,995
Changes in estimates of existing obligations (2) (38,470) (85,514)
Property dispositions/obligations settled (6,132) (5,795)
Ending balance 363,956 275,452
Annual funding obligation 36,400  
Asset retirement obligation [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Changes in estimates of existing obligations (2) (38,000) $ (86,000)
Accretion (1) 87,000  
External ARO trust [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Annual funding obligation $ 36,400