Leases - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2015	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2016	Dec. 31, 2018
Operating Leased Assets [Line Items]
Operating lease costs (gross lease expense)		$ 3,200	$ 1,200	$ 9,300	$ 3,000
Variable lease costs		1,500		5,000
Sublease income		$ 1,900		$ 5,600
Operating lease, Weighted average remaining operating lease term		8 years 8 months 1 day		8 years 8 months 1 day
Operating lease, weighted average discount rate, percent		7.40%		7.40%
Financing obligation on leased facility		$ 0		$ 0			$ 92,009
San Mateo Facility
Operating Leased Assets [Line Items]
Operating lease expiration period	Dec. 31, 2028
Capitalized construction costs						$ 71,800
Leasehold improvement costs reimbursed by landlord						$ 14,300
Financing obligation on leased facility							$ 92,000