Supplemental Disclosures to the Statements of Cash Flows - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 59,938	$ 56,352
Restricted cash (included in other current assets)	8,070	9,108
Cash, cash equivalents, and restricted cash	$ 68,008	$ 65,460	$ 77,333	$ 95,299