Business - Additional Information (Detail) - USD ($) $ in Thousands				1 Months Ended		3 Months Ended		6 Months Ended
	Jul. 23, 2018	Jul. 18, 2018	Apr. 05, 2017	Jul. 31, 2018	May 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Nature Of Business [Line Items]
Date of formation								Jul. 02, 2004
Proceeds from equity and debt financing						$ 291,100		$ 291,100
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						131,961		131,961
Accumulated deficit						254,302		254,302		$ 220,341
Net loss						$ 17,194	$ 9,300	33,961	$ 31,504
Net cash provided by (used in) operating activities								$ 22,552	$ (33,563)
Subsequent Event [Member]
Nature Of Business [Line Items]
Proceeds from issuance of common stock net of underwriting discounts and offering costs	$ 92,026
Subsequent Event [Member] | Follow-on Public Offering [Member]
Nature Of Business [Line Items]
Proceeds from issuance of common stock net of underwriting discounts and offering costs				$ 92,026
Common Stock [Member] | Follow-on Public Offering [Member]
Nature Of Business [Line Items]
Common stock, shares issued			5,117,500						5,117,500
Proceeds from issuance of common stock net of underwriting discounts and offering costs			$ 86,224
Common Stock [Member] | Subsequent Event [Member]
Nature Of Business [Line Items]
Common stock, shares issued		5,175,000
Common Stock [Member] | Subsequent Event [Member] | Follow-on Public Offering [Member]
Nature Of Business [Line Items]
Common stock, shares issued	5,175,000			5,175,000
Proceeds from issuance of common stock net of underwriting discounts and offering costs	$ 92,026
Common Stock [Member] | Subsequent Event [Member] | Underwriter's Over-Allotment [Member]
Nature Of Business [Line Items]
Common stock, shares issued	675,000	675,000		675,000
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