Business - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended						9 Months Ended
	Jul. 29, 2019	Jul. 31, 2019	May 31, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Nature Of Business [Line Items]
Date of formation										Jul. 02, 2004
Proceeds from equity and debt financing				$ 519,700						$ 519,700
Upfront and milestone payments received in connection with license agreements				88,900						88,900
Proceeds from sale of common stock in connection with license agreement											$ 14,556
Unrestricted cash and cash equivalents and marketable securities				249,074						249,074
Accumulated deficit				372,116						372,116		$ 294,354
Net loss				$ 32,842	$ 22,960	$ 21,960	$ 19,400	$ 17,194	$ 16,767	77,762	53,361
Net cash (used in) provided by operating activities										$ (78,059)	$ 2,060
Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued		6,325,000
Common Stock [Member] | Follow-on Public Offering [Member]
Nature Of Business [Line Items]
Common stock, shares issued				6,325,000			5,175,000
Common Stock [Member] | Underwriter's Over-Allotment [Member]
Nature Of Business [Line Items]
Common stock, shares issued	825,000	825,000
Proceeds from issuance of common stock net	$ 136,525
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Nature Of Business [Line Items]
Proceeds from sale of common stock in connection with license agreement			$ 14,556
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] | Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued			1,174,827