License, Collaboration and Other Significant Agreements - Vifor Pharma License Agreement (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended	9 Months Ended
	Feb. 14, 2020	Apr. 08, 2019	May 12, 2017	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Common stock, shares issued (in shares)					143,328,652	121,674,568
Deferred revenue, net of current portion					$ 33,660	$ 33,120
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]
Potential milestone revenue		$ 25,000	$ 25,000
License agreement, notice period for termination					12 months
Common stock, shares 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
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