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Supplemental Financial Information - Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Asset retirement obligation, beginning of year $ 1,248 $ 1,188
Accretion 64 60
Asset retirement obligations, end of year $ 1,312 $ 1,248