Cash and Cash Equivalents and Short Term Investments - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 78,592	$ 99,780	$ 119,423	$ 57,613
Cash
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	5,522	29,915
Money market funds
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 73,070	$ 69,865