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Significant Accounting Policies - Concentrations of Credit Risk and Other Risks and Uncertainties (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Accounting Policies [Abstract]          
Cash and cash equivalents $ 22,936 $ 19,623 $ 14,068 $ 18,731 $ 11,332