Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Gross rent expense		$ 1,200	$ 1,000	$ 3,000	$ 3,400
Financing obligation on leased facility		92,349		92,349		$ 93,385
Standby letters of credit issued		7,800		7,800
San Mateo Facility
Other Commitments [Line Items]
Finance lease expiration period	Dec. 31, 2028
Capitalized construction costs							$ 71,800
Leasehold improvement costs reimbursed by landlord						6,800	$ 14,300
Financing obligation on leased facility		$ 92,300		$ 92,300		$ 93,400