Commitments and Contingencies - Narrative (Details)	1 Months Ended		9 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2017	Dec. 31, 2017 USD ($) building	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Long-term Purchase Commitment [Line Items]
Number of buildings | building				4
Incentive from lessor		$ 1,300,000		$ 900,000	$ 1,300,000
Rent expense				3,200,000	2,900,000	$ 2,900,000
Sublease income				1,400,000	1,200,000	$ 600,000
Asset retirement obligations		400,000		200,000	400,000
Letters of credit		$ 800,000		700,000	800,000
Capital leases, term of contract	3 years	3 years
Capital lease obligations incurred	$ 300,000	$ 400,000
Non-current portion of lease incentive obligation
Long-term Purchase Commitment [Line Items]
Incentive from lessor		900,000		500,000	900,000
Indemnification agreement
Long-term Purchase Commitment [Line Items]
Accruals for expenses related to indemnification issues		$ 0		0	$ 0
Minimum
Long-term Purchase Commitment [Line Items]
Lease expiration date			Jan. 31, 2020
Maximum
Long-term Purchase Commitment [Line Items]
Lease expiration date			Jan. 31, 2022
Redwood City facility
Long-term Purchase Commitment [Line Items]
Decrease in asset retirement obligations				200,000
Selling, general and administrative | Redwood City facility
Long-term Purchase Commitment [Line Items]
Gain on derecognition				$ 200,000