Significant Contracts - Additional Information (Detail)				1 Months Ended	3 Months Ended							9 Months Ended		12 Months Ended
		Feb. 12, 2016 USD ($)	Oct. 26, 2015 USD ($)	Oct. 31, 2019 Option License	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) Option	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-current deferred revenue						$ 11,871,000						$ 11,871,000		$ 57,780,000
Collaboration revenue	[1]					211,928,000			$ 563,000			212,574,000	$ 3,009,000
Current deferred revenue						47,725,000						47,725,000		102,000
Exercise of vested options						2,338,000	$ 1,254,000	$ 1,832,000	1,841,000	$ 3,729,000	$ 2,656,000
Research and development expense	[2]					57,246,000			39,820,000			$ 130,601,000	84,972,000
Date of joint venture agreement												Dec. 19, 2015
Equity method investment															$ 36,500,000
Unrealized equity method losses						3,430,000			1,012,000			$ 5,467,000	3,256,000		$ 36,500,000
Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue						100,000			900,000			500,000	2,000,000.0
Research and development expense						100,000			1,200,000			$ 700,000	2,400,000
Date of formation of joint venture entity												Feb. 12, 2016
Equity method investment, ownership percentage		50.00%
Cash contribution		$ 100,000
Other payments due to company from joint venture entity		0
Unrealized equity method losses						3,300,000			1,200,000			$ 5,500,000	2,200,000
Net loss of joint venture						22,600,000			13,500,000			54,900,000	25,800,000
Unrecognized equity method losses in excess of Company's interest												70,100,000		$ 45,300,000
Collaboration Target Options [Member] | Scenario Forecast [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue					$ 76,700,000
Exercise of vested options					30,000,000
Deferred revenue					46,700,000
Collaboration Target Options [Member] | Subsequent Event [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of collaboration target options | Option				3
License and Service [Member] | Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue		35,000,000.0
Royalty [Member] | Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue		$ 0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Nonrefundable upfront payment received			$ 75,000,000.0									175,000,000.0
Transaction price												268,600,000
Non-current deferred revenue						11,900,000						11,900,000
Non-cash consideration received												10,000,000.0
Collaboration revenue						211,900,000			100,000			212,100,000	500,000
Current deferred revenue						47,700,000						47,700,000
Transaction price allocated to remaining performance obligations						59,600,000						59,600,000
Research and development expense						7,500,000			8,800,000			21,300,000	25,400,000
Reimbursements from research and license agreements						3,800,000			$ 3,700,000			11,800,000	$ 10,400,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Research and Development [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Variable consideration received												25,000,000.0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Collaboration Target Options [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment receivable						420,000,000.0						420,000,000.0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | DM1 R&D Services [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												1,700,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | DM1 R&D Services [Member] | Research and Development [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Variable consideration received												800,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | DMD License [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												224,600,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | DM1 License [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												76,200,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Specified Target Option Material Right [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												17,500,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Collaboration Target Options Material Rights One [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												25,000,000.0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Collaboration Target Options Material Rights Two [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												22,200,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Collaboration Target Options Material Rights Three [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												22,200,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Joint Development Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Up-front payment received												$ 7,000,000.0
Agreement description												In connection with the JDA, the Company received a $7.0 million up-front payment from Vertex and subsequently received a one-time low seven-digit milestone payment upon the dosing of the second patient in a clinical trial with the initial product candidate.
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Exercise of Exclusive Option [Member] | Collaboration Target Options [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment receivable						10,000,000						$ 10,000,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | 2019 Collaboration Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Nonrefundable upfront payment received												175,000,000
Maximum potential future payments												$ 825,000,000
Percentage of exchange payment of research and development costs												50.00%
Collaborative arrangement, license rights reacquired by waiving payment owed												$ 10,000,000.0
Non-cash consideration expensed upon execution of collaborative agreement												10,000,000.0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | 2019 Collaboration Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contract asset						25,000,000.0						25,000,000.0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | 2015 Collaboration Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment receivable						395,000,000						$ 395,000,000
Percentage of exchange payment of research and development costs												50.00%
Number of options would not be exercised under agreement | Option												1
Number of remaining options under agreement | Option												4
Non-current deferred revenue						$ 57,800,000						$ 57,800,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | 2015 Collaboration Agreement [Member] | Collaborative Arrangement Material Rights Fourth Exclusive License [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration												$ 10,000,000.0
Vertex Pharmaceuticals Inc [Member] | 2015 Collaboration Agreement [Member] | Scenario Forecast [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount in option exercise payments to be received					$ 30,000,000.0
Vertex Pharmaceuticals Inc [Member] | 2015 Collaboration Agreement [Member] | Subsequent Event [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of exclusive license targets | License				3

[1] Including the following revenue from a related party, see Notes 6 & 11

[2] Including the following research and development expense with a related party, see Notes 6 & 11