Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) agreement letter	Dec. 31, 2017 USD ($) letter	Dec. 31, 2016 USD ($)
Commitment And Contingencies [Line Items]
Number of non-cancellable operating lease agreements | agreement	4
Total	$ 27,939
2019	3,569
2020	4,683
2021	4,914
2022	5,052
2023	5,194
Rent expense	$ 4,200	$ 3,700	$ 3,600
Letters of credit (in letter) | letter	3	2
Letters of credit outstanding, amount	$ 800	$ 800
Accrued settlement costs			1,100
Office Space in San Francisco
Commitment And Contingencies [Line Items]
Sublease income		$ 900	$ 900
Total	15,700
Office Space in San Francisco | Minimum
Commitment And Contingencies [Line Items]
2019	1,000
2020	1,000
2021	1,000
2022	1,000
2023	1,000
2024	1,000
Office Space in San Francisco | Maximum
Commitment And Contingencies [Line Items]
2019	2,200
2020	2,200
2021	2,200
2022	2,200
2023	2,200
2024	2,200
Office Space in Chicago
Commitment And Contingencies [Line Items]
Total	10,300
Office Space in Chicago | Minimum
Commitment And Contingencies [Line Items]
2019	500
2020	500
2021	500
2022	500
2023	500
2024	500
Office Space in Chicago | Maximum
Commitment And Contingencies [Line Items]
2019	2,000
2020	2,000
2021	2,000
2022	2,000
2023	2,000
2024	$ 2,000