Commitments and Contingencies - Operating Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Future minimum payment commitments required under non-cancellable office space, co-location agreements and third-party licenses
2018	$ 5,794
2019	6,444
2020	6,492
2021	5,400
2022	4,922
Thereafter	18,127
Total minimum operating commitments	47,179
Less: non-cancelable sublease income	(5,526)
Total operating commitments	41,653
Legal Contingencies
Reserves recorded	0	$ 0
Former Headquarters New York, NY
Letters of Credit
Outstanding letters of credit	450	450
Office Space Mountain View, CA
Letters of Credit
Outstanding letters of credit	320	320
Headquarters New York, NY
Letters of Credit
Outstanding letters of credit	2,332	2,332
Operating income
Operating Commitments
Rent expense	3,033	3,008	$ 2,849
Other income, net
Operating Commitments
Rent expense	1,908	1,292
Rent income	$ 2,107	$ 1,686