Revenue Recognition - Schedule of Changes in Contract Assets and Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period		$ 0
Balance at end of period	$ 5,876	5,876
Amgen Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	6,250	18,750
Payments made for the Co-Invest Option	(6,250)	(18,750)
Balance at end of period	0	0
2016 Astellas Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	10,375	19,413
Reduction for services performed	(5,562)	(12,865)
Cash received in advance of services performed	1,063	(672)
Balance at end of period	5,876	5,876
2014 Astellas Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Balance at beginning of period	1,882	6,288
Reduction for services performed	(1,882)	(6,288)
Balance at end of period	$ 0	$ 0