Collaboration and License Agreements - General Information (Details) $ in Thousands			1 Months Ended				3 Months Ended				12 Months Ended
	Oct. 15, 2019 USD ($) item	Jun. 01, 2018 USD ($) item	Oct. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2016 USD ($) item	Mar. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($) item	Mar. 31, 2019 USD ($)	Mar. 31, 2016 USD ($) item	Dec. 31, 2019 USD ($)
Collaboration Agreements
Collaboration revenue							$ 6,167		$ 730
Current portion of deferred revenue							$ 8,690	$ 6,160			$ 6,160
Roche | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01
Collaboration Agreements
Research and development services, performance period							5 years
Collaboration | Clementia
Collaboration Agreements
Non-refundable upfront payment received	$ 25,000
Eligible milestone payments	510,000
Cash milestone payment	20,000
Milestone other payment	490,000
Purchase specified manufacturing inventory	$ 1,500
Number of performance obligations | item	3
Purchase of existing manufacturing inventory	$ 1,200
Purchase of in-process manufacturing inventory	$ 300
Number of material promises | item	4
Initial cash payment	$ 46,500
Collaboration revenue							$ 0		0		46,200
Deferred revenue							0		0
Collaboration | C Stone
Collaboration Agreements
Non-refundable upfront payment received		$ 40,000
Eligible milestone payments		$ 346,000
Number of performance obligations | item		3
Licensed product term from first commercial sale		12 years
Number of materials components | item		2
Number of option rights to obtain exclusive license | item		3
Number of potential collaboration programs | item		3
Number of material promises | item		6
Number of collaboration programs with exclusive commercialization rights for licensed products | item		3
Initial cash payment		$ 40,000
Collaboration revenue							2,134
Deferred revenue							$ 0	$ 0			$ 0
Revenue recognized, change in contract liability									40,000
Collaboration | C Stone | Development and regulatory milestones
Collaboration Agreements
Eligible milestone payments		118,500
Number of potential collaboration programs | item							1
Collaboration revenue							$ 2,000
Collaboration | C Stone | Sales-based milestones
Collaboration Agreements
Eligible milestone payments		$ 227,500
Collaboration | Roche
Collaboration Agreements
Non-refundable upfront payment received						$ 45,000
Number of small molecule therapeutics targeting kinases | item						5				5
Number of option rights to obtain exclusive license | item						5				5
Number of potential collaboration programs | item						4		4
Initial cash payment					$ 45,000	$ 45,000
Collaboration revenue							575		730
Deferred revenue							45,500
Total eligible contingent option fees and milestone payments						940,000				$ 940,000
Eligible option fees and milestones prior to licensing for all potential collaboration programs						$ 190,000
Current portion of deferred revenue							8,700
Achieved research milestone payment received which will be recognized over period of performance			$ 8,000	$ 10,000
Revenue recognized, change in contract liability							$ 989		$ 987
Collaboration | Roche | Prior To Exercise License Right Option
Collaboration Agreements
Written termination notice period						120 days
Collaboration | Roche | Exercise Of License Right Option
Collaboration Agreements
Number of collaboration programs with exclusive commercialization rights for licensed products | item						2
Number of collaboration programs with specified commercial rights for each party | item						2
Collaboration | Roche | Exercise Of License Option Product Not Commercially Sold
Collaboration Agreements
Written termination notice period						120 days
Collaboration | Roche | Exercise Of License Option Product Commercially Sold
Collaboration Agreements
Written termination notice period						180 days