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- Schedule of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2016
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]      
Cash and cash equivalents $ 43,239   $ 18,574
Restricted cash included in other long-term assets 21,594   $ 22,127
Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 64,833 $ 40,701