License, Collaboration and Other Significant Agreements - Vifor Pharma License Agreement (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Feb. 14, 2020	May 12, 2017	Mar. 31, 2020	Sep. 30, 2021	Dec. 31, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock, issued (in shares)				174,551,989	148,074,085
Deferred revenue, net of current portion				$ 27,965	$ 25,345
Vifor (International) Ltd. | Research and Development Expense
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Payments for closing Priority Review Voucher purchase			$ 10,000
Vifor (International) Ltd. | License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock, issued (in shares)		3,571,429
Sale of stock, price per share (in dollars per share)		$ 14.00
Proceeds from common stock sold		$ 50,000
Premium over the closing stock price of common stock (in dollars per share)		$ 12.69
Premium amount over the closing stock price of common stock		$ 4,700
Potential milestone revenue		25,000
Deferred revenue, net of current portion		$ 4,700
Vifor (International) Ltd. | Priority Review Voucher Letter Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Payments for closing Priority Review Voucher purchase	$ 10,000