Commitments and Contingencies - Letters of Credit (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Former Headquarters New York, NY, 1
Line of Credit Facility [Line Items]
Outstanding letters of credit	$ 450	$ 450
Office Space Mountain View, CA
Line of Credit Facility [Line Items]
Outstanding letters of credit	320	320
Former Headquarters New York, NY, 2
Line of Credit Facility [Line Items]
Outstanding letters of credit	2,332	2,332
Current Headquarters New York, NY
Line of Credit Facility [Line Items]
Outstanding letters of credit	$ 633	$ 633