Commitments and Contingencies - San Francisco Office Lease (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2014	Dec. 31, 2016	Dec. 31, 2017
Operating Leased Assets [Line Items]
Property and equipment net, capitalized		$ 66,941	$ 70,710
Lease Financing Obligation Current and Noncurrent Amount		30,900	31,400
Lease
Operating Leased Assets [Line Items]
Expected cumulative base rent over initial lease term			77,096
San Francisco Office
Operating Leased Assets [Line Items]
Total future minimum lease commitments period	10 years
Standby letter of credit	$ 800
Lease Financing Obligation Current and Noncurrent Amount		$ 6,800	6,900
Estimated useful life		40 years
San Francisco Office | Capitalized facility leases
Operating Leased Assets [Line Items]
Property and equipment net, capitalized		$ 8,700	$ 8,700
San Francisco Office | Debt Redemption Period August 1, 2017 to August 1, 2020
Operating Leased Assets [Line Items]
Amount of annual reduction of letter of credit	200
San Francisco Office | Lease
Operating Leased Assets [Line Items]
Expected cumulative base rent over initial lease term	$ 7,000