Commitments and Contingencies - Operating Leases (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) agreement	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) agreement	Sep. 30, 2018 USD ($)	Jan. 31, 2019 office_space agreement	Dec. 31, 2018 office_space
Lessee, Lease, Description [Line Items]
Number of non-cancellable operating lease agreements | agreement			4
Lease payment, year 2	$ 5,921		$ 5,921
Lease payment, year 3	6,405		6,405
Lease payment, year 4	6,588		6,588
Lease payment, year 5	6,776		6,776
Lease payment, thereafter	13,137		13,137
Rent expense	1,200	$ 1,100	3,900	$ 3,000
Santa Clara
Lessee, Lease, Description [Line Items]
Total minimum lease payments	14,300		14,300
Santa Clara | Minimum
Lessee, Lease, Description [Line Items]
Total minimum lease payments	1,400		1,400
Lease payment, year 1	1,400		1,400
Lease payment, year 2	1,400		1,400
Lease payment, year 3	1,400		1,400
Lease payment, year 4	1,400		1,400
Lease payment, year 5	1,400		1,400
Lease payment, thereafter	1,400		1,400
Santa Clara | Maximum
Lessee, Lease, Description [Line Items]
Total minimum lease payments	1,800		1,800
Lease payment, year 1	1,800		1,800
Lease payment, year 2	1,800		1,800
Lease payment, year 3	1,800		1,800
Lease payment, year 4	1,800		1,800
Lease payment, year 5	1,800		1,800
Lease payment, thereafter	1,800		1,800
San Francisco
Lessee, Lease, Description [Line Items]
Total minimum lease payments	15,700		15,700
San Francisco | Minimum
Lessee, Lease, Description [Line Items]
Total minimum lease payments	1,000		1,000
Lease payment, year 1	1,000		1,000
Lease payment, year 2	1,000		1,000
Lease payment, year 3	1,000		1,000
Lease payment, year 4	1,000		1,000
Lease payment, year 5	1,000		1,000
Lease payment, thereafter	1,000		1,000
San Francisco | Maximum
Lessee, Lease, Description [Line Items]
Total minimum lease payments	2,200		2,200
Lease payment, year 1	2,200		2,200
Lease payment, year 2	2,200		2,200
Lease payment, year 3	2,200		2,200
Lease payment, year 4	2,200		2,200
Lease payment, year 5	2,200		2,200
Lease payment, thereafter	2,200		2,200
Chicago
Lessee, Lease, Description [Line Items]
Total minimum lease payments	10,300		$ 10,300
Accelerated depreciation	$ 300
Number of sublease agreements | agreement	2		2		2
Number of leased spaces | office_space					2	2
Chicago | Minimum
Lessee, Lease, Description [Line Items]
Total minimum lease payments	$ 500		$ 500
Lease payment, year 1	500		500
Lease payment, year 2	500		500
Lease payment, year 3	500		500
Lease payment, year 4	500		500
Lease payment, year 5	500		500
Lease payment, thereafter	500		500
Chicago | Maximum
Lessee, Lease, Description [Line Items]
Total minimum lease payments	2,000		2,000
Lease payment, year 1	2,000		2,000
Lease payment, year 2	2,000		2,000
Lease payment, year 3	2,000		2,000
Lease payment, year 4	2,000		2,000
Lease payment, year 5	2,000		2,000
Lease payment, thereafter	$ 2,000		$ 2,000