License Agreements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		152 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revised agreement potential milestone receivable			$ 55,000
Adjustment to accumulated deficit	$ 1,041,336		1,041,336		$ 1,041,336		$ 998,538
License revenue	1,644	$ 1,028	2,773	$ 2,343
License expense	$ 987	$ 617	$ 1,664	$ 1,406
Licensing Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payments to acquire right for development and marketing of Ferric Citrate					$ 11,600
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Adjustment to accumulated deficit						$ 600