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Asset Retirement Obligations (ARO) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation Roll Forward      
Balance at beginning of period $ 22,245 $ 17,130  
Accretion expense 1,521 1,200 $ 1,096
Liabilities incurred 579 3,915  
Balance at end of period $ 24,345 $ 22,245 $ 17,130