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Cash Deposits and Other Assets (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]    
Insurance collateral deposits $ 4,824 $ 5,032
State collateral deposits 62 143
Materials and supplies 344 354
Total $ 5,230 $ 5,529