Summary of Significant Accounting Policies - Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 130,155	$ 35,470
Restricted cash included in other long-term assets	827	794
Total cash and cash equivalents and restricted cash included in the Condensed Statements of Cash Flows	$ 130,982	$ 36,264	$ 235,407	$ 152,656