Convertible Preferred Stock and Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2021	Nov. 30, 2016	Jun. 30, 2020	Sep. 30, 2021	Dec. 31, 2019	Dec. 31, 2020	May 02, 2017
Convertible preferred stock
Preferred stock, par value (in dollars per share)				$ 0.001		$ 0.001
Employee And Non Employee Stock Option
Convertible preferred stock
Unrecognized compensation cost related to outstanding options				$ 56.5
Expected recognition weighted average period of unrecognized compensation cost				2 years 6 months
Restricted Stock Units (RSUs)
Convertible preferred stock
Expected recognition weighted average period of unrecognized compensation cost				3 years 3 months 18 days
Unrecognized compensation cost related to unvested restricted shares				$ 92.1
Pre-Funded Warrants
Convertible preferred stock
Number of shares to be purchased	257,310			257,310
Purchase price of prefunded warrants	$ 85.499
Exercise price of warrants	$ 0.001
Terms of exercise	The Pre-Funded Warrants are exercisable at any time after the date of issuance. A holder of Pre-Funded Warrants may not exercise the Pre-Funded Warrant if the holder, together with its affiliates, would beneficially own more than 9.99% of the number of shares of the Company’s common stock outstanding immediately after giving effect to such exercise. A holder of Pre-Funded Warrants may increase or decrease this percentage not in excess of 19.99% by providing at least 61 days’ prior notice to the Company
Maximum percentage of common stock ownership together with affiliates allowable after exercise of prefunded warrants	9.99%
Maximum percentage of common stock ownership after change upon notice	19.99%
Common Stock
Convertible preferred stock
Common stock issued			7,108,796
Conversion of preferred shares to common stock					125,000
Non-Voting Class A Preferred Stock | Redmile Group, LLC and Affiliates
Convertible preferred stock
Terms of conversion				The Class A Preferred were purchased exclusively by entities affiliated with Redmile Group, LLC (collectively, Redmile). The terms of the CoD prohibited Redmile from converting the Class A Preferred into shares of the Company’s common stock if, as a result of conversion, Redmile, together with its affiliates, would own more than 9.99% of the Company’s common stock then issued and outstanding (the Redmile Percentage Limitation), which percentage could change at Redmile’s election upon 61 days’ notice to the Company to (i) any other number less than or equal to 19.99% or (ii) subject to approval of the Company’s stockholders to the extent required in accordance with the Nasdaq Global Market rules, any number in excess of 19.99%. On May 2, 2017, the Company’s stockholders approved the issuance of up to an aggregate of 14,097,745 shares of common stock upon the conversion of the outstanding shares of Class A Preferred. As a result, Redmile has the right to increase the Redmile Percentage Limitation to any percentage in excess of 19.99% at its election.
Non-Voting Class A Preferred Stock | Maximum
Convertible preferred stock
Number of shares to be issued upon conversion							14,097,745
Non-Voting Class A Preferred Stock | Maximum | Redmile Group, LLC and Affiliates
Convertible preferred stock
Percentage of common stock ownership upon preferred stock conversion		9.99%
Preferred shares converted into common stock percentage of ownership change upon notice		19.99%
Class A Convertible Preferred Shares
Convertible preferred stock
Preferred stock, issued shares				2,794,549		2,794,549
Conversion of preferred shares to common stock					(25,000)
November 2016 Placement | Common Stock
Convertible preferred stock
Share issue price (in dollars per share)		$ 2.66
Common stock issued		7,236,837
November 2016 Placement | Non-Voting Class A Preferred Stock
Convertible preferred stock
Preferred stock, issued shares		2,819,549
Share issue price (in dollars per share)		$ 13.30
Number of shares to be issued upon conversion		5
Preferred stock, par value (in dollars per share)		$ 0.001
Conversion price		$ 2.66