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Significant Collaboration and Licensing Agreements - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended				3 Months Ended			12 Months Ended	61 Months Ended
	Jan. 31, 2019	Feb. 28, 2017	Mar. 31, 2015	Jun. 30, 2010	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2017	Mar. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues					$ 237.1		$ 138.4
Acquired in-process research and development							113.1
Equity securities fair value					39.4				$ 39.4	$ 55.9
Collaboration revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues					6.0		2.0
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments received				$ 75.0
Collaborative arrangement right				We will be entitled to a percentage of worldwide sales of GnRH Compounds for the longer of ten years or the life of the related patent rights.
Collaboration termination notice period				180 days
AbbVie \| Development and Regulatory Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential development and regulatory milestone payment receipts					480.0
Revenues					135.0
AbbVie \| Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential commercial milestone payment receipts				$ 50.0
B I A L
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Acquired in-process research and development		$ 30.0
Potential payments for regulatory milestones		40.0
Payments made for regulatory milestones					20.0				20.0
Potential payments for commercial milestones		$ 75.0
Voyager Therapeutics \| Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration termination notice period	180 days
Upfront payments made	$ 115.0
Equity securities cost	$ 50.0
Share price	$ 11.9625
Equity securities fair value	$ 54.7
Voyager Therapeutics \| Collaboration and License Agreement \| IPR&D
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Acquired in-process research and development						$ 5.0	$ 113.1
Voyager Therapeutics \| Collaboration and License Agreement \| Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares owned	4.2
Voyager Therapeutics \| Collaboration and License Agreement \| Development, Regulatory and Commercial Milestone Payment
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payments	$ 1,700.0
Xenon \| Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments made										30.0
Equity securities cost										$ 20.0
Share price										$ 14.196
Xenon \| Collaboration and License Agreement \| Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares owned										1.4
Xenon \| Collaboration and License Agreement \| Development, Regulatory and Commercial Milestone Payment
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payments										$ 1,700.0
Mitsubishi Tanabe
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments received			$ 30.0
Patent rights period			the longer of ten years or the life of the related patent rights.
Collaboration termination notice period			180 days
Potential milestone payment receipts			$ 85.0
Deferred revenue					8.1				8.1
Deferred revenue recognized					$ 1.3
Mitsubishi Tanabe \| Collaboration revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues									$ 21.9
Mitsubishi Tanabe \| Development and Regulatory Milestone Payments \| Collaboration revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues								$ 15.0