License and Collaboration Agreements - Additional Information (Detail) - USD ($) $ in Thousands				3 Months Ended		9 Months Ended
	Apr. 04, 2019	Feb. 27, 2019	May 03, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent consideration liability of acquisition				$ 5,200		$ 5,200
Myonexus Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Final exercise price		$ 165,000
Additional transaction fees		8,800		165,000		165,000
Additional development milestone payments to be paid		$ 200,000
Date of acquisition	Apr. 04, 2019
Contingent consideration liability of acquisition				4,500		4,500
Warrant Agreement [Member] | Myonexus Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements			$ 60,000
Cash consideration under agreement excluding contingent payments			$ 200,000
License and Collaboration Agreements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone and up-front expenses recognized as research and development expense				10,200	$ 18,000	25,700	$ 78,000
License and Collaboration Agreements [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory, commercial milestone and up-front royalty payments				1,508,700		1,508,700
License and Collaboration Agreements [Member] | Summit [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contractual obligation				$ 500		$ 500