Research and Development Arrangements - Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total Revenue	$ 10,641	$ 6,180	$ 22,124	$ 13,386
Astellas [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development revenues	8,526	2,112	16,791	8,810
Total Revenue	10,441	2,430	21,924	15,517
Astellas [Member] \| License Revenues [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
License revenues	$ 1,915	$ 318	$ 5,133	$ 6,707

The amount of compensation earned (contract income) under a research and development arrangement accounted for as a contract to perform research and development for others.

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