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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]    
Balance, beginning of period $ 314,970 $ 242,224
Liabilities incurred 15,110 15,114
Liabilities settled (4,981) (4,338)
Accretion expense 16,839 13,918
Revisions in estimates 31 48,052
Balance, end of period 341,969 314,970
Asset retirement obligation costs [Member]    
Regulatory Assets [Line Items]    
Accretion expense, including asset retirement obligations $ 15,600 $ 12,700