Significant Contracts - Additional Information (Detail)								1 Months Ended		2 Months Ended	3 Months Ended								12 Months Ended
		Dec. 13, 2019 USD ($) Product	Dec. 18, 2018 USD ($)	Nov. 15, 2018 USD ($) shares	Sep. 24, 2018 USD ($) shares	Sep. 17, 2018 USD ($) Installment	Oct. 26, 2015 USD ($)	Oct. 31, 2019 License	Dec. 18, 2018 USD ($)	Nov. 14, 2018 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) Option		Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 12, 2019	Mar. 31, 2016	Feb. 12, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-current deferred revenue											$ 11,776,000				$ 57,780,000				$ 11,776,000		$ 57,780,000
Collaboration revenue											77,016,000	$ 211,928,000	$ 318,000	$ 328,000	115,000	$ 563,000	$ 1,088,000	$ 1,358,000	289,590,000	[1]	3,124,000	[1]	$ 40,997,000	[1]
Current deferred revenue											960,000				102,000				960,000		102,000
Research and development expense	[2]																		$ 179,362,000		113,773,000		69,800,000
Date of joint venture agreement																			Dec. 19, 2015
Operating expenses											66,033,000	$ 72,765,000	$ 55,301,000	$ 48,751,000	45,343,000	$ 49,995,000	$ 38,374,000	$ 28,355,000	$ 242,850,000		162,067,000		105,645,000
Stock-based compensation expense																			49,524,000		39,260,000		20,636,000
Issuance of common shares, value																			414,789,000		307,053,000
Net proceeds from issuance of common shares																			415,019,000		307,053,000
Other (expense) income																			26,033,000		(1,210,000)		(197,000)
Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense																			700,000		3,800,000		4,500,000
Reimbursements from research and license agreements																			$ 200,000		900,000		4,400,000
Date of formation of joint venture entity																			Feb. 12, 2016
Equity method investment, ownership percentage																									50.00%	50.00%	50.00%
Stock-based compensation expense																			$ 5,500,000		4,300,000		1,800,000
Unrecognized equity method losses in excess of Company's interest																			72,000,000.0		45,300,000
Net loss of joint venture		$ 58,800,000																			52,500,000		36,200,000
Business combination, consideration transferred																			41,000,000.0
Net assets acquired											$ 16,000,000.0								16,000,000.0
Fair value of controlling interest obtained																			32,000,000.0
Research and Development [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock-based compensation expense																			23,273,000		17,557,000		8,800,000
License and Service [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue																			$ 500,000		2,500,000		4,800,000
Non-Exclusive Research License [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price allocated to remaining performance obligations							$ 500,000
Non-Exclusive Research License [Member] | Valuation, Market Approach
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for collaboration agreement							1,000,000.0
Retirement Agreement [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percentage of partnership interests		100.00%
Partnership outstanding maximum exchange amount		$ 22,000,000.0
Operating expenses		$ 6,000,000.0
2019 Option Agreement [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Equity method investment, ownership percentage											50.00%								50.00%
Number of products | Product		2
Non-refundable one-time option payment		$ 20,000,000.0
Business combination, consideration transferred																			$ 25,000,000.0
Payment to acquire business																			$ 16,000,000.0
Number of options exercised under agreement | Option																			2
2019 Option Agreement [Member] | Research And Development Services | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Fair value of obligations											$ 20,200,000								$ 20,200,000
2019 Option Agreement [Member] | Future Delivery of up to Two Options [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Fair value of obligations											4,800,000								4,800,000
2019 Option Agreement [Member] | Research and Development [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Business combination, consideration transferred																			20,200,000
2019 Option Agreement [Member] | Certain Options [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Business combination, consideration transferred																			4,800,000
Co-commercialization Agreement [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percentage of research and development costs		50.00%
Percentage sharing of profit (loss) from sale of product		50.00%
2015 Casebia Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Equity method investment											0								0
2019 Casebia Agreement [Member] | Casebia Therapeutics Limited Liability Partnership [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Equity method investment											0								0
Fair value of assets and liabilities											16,000,000.0								16,000,000.0
2019 Casebia Agreement [Member] | Other Current Liabilities [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research obligation											11,000,000.0								11,000,000.0
2019 Casebia Agreement [Member] | Other Long-term Liabilities [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research obligation											14,000,000.0								14,000,000.0
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											11,800,000				57,800,000				11,800,000		57,800,000
Non-cash consideration received																			10,000,000.0
Transaction price allocated to remaining performance obligations											12,700,000								12,700,000
Collaboration revenue																			30,000,000.0
Revenue																			289,100,000		600,000		36,200,000
Current deferred revenue											900,000				$ 100,000				900,000		100,000
Research and development expense																			29,200,000		20,200,000		9,900,000
Reimbursements from research and license agreements																			15,900,000		13,800,000		0.0
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							19,300,000												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
Milestone payment receivable											410,000,000.0								410,000,000.0
Transaction price allocated to remaining performance obligations											46,700,000								46,700,000
Revenue																			76,700,000
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
Transaction price allocated to remaining performance obligations											1,100,000								1,100,000
Revenue																			100,000
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
Transaction price allocated to remaining performance obligations											151,100,000								151,100,000
Revenue																			289,100,000
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
Transaction price allocated to remaining performance obligations											51,300,000								51,300,000
Revenue																			202,400,000
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
Transaction price allocated to remaining performance obligations											11,800,000								11,800,000
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] | Waiving of Vertex’s Material Right [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue																			6,700,000
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] | Collaborative Arrangement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Nonrefundable upfront payment received																			175,000,000.0
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] | Collaborative Arrangement [Member] | Prepaid Expenses and Other Current Assets
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											395,000,000.0								$ 395,000,000.0
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
Transaction price allocated to remaining performance obligations											6,700,000								6,700,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Revenue Related to Three Collaboration Target Options Material Right [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue																			10,000,000.0
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Beta Globin
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration																							$ 30,300,000
Vertex Pharmaceuticals Incorporated and Certain of its Subsidiaries [Member] | Non-Exclusive Research License [Member] | Research And Development Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue																			$ 100,000		600,000
Vertex Pharmaceuticals Inc [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount in option exercise payments received											30,000,000.0
Estimated standalone selling price for single collaboration							19,300,000
Transaction price allocated to remaining performance obligations							19,300,000
Estimated standalone selling price for collaboration agreement							118,600,000
Vertex Pharmaceuticals Inc [Member] | Collaboration Target Options [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price allocated to remaining performance obligations							57,700,000
Vertex Pharmaceuticals Inc [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							45,600,000
Vertex Pharmaceuticals Inc [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							38,400,000
Vertex Pharmaceuticals Inc [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							17,300,000
Vertex Pharmaceuticals Inc [Member] | Rights Four [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration							17,300,000
Vertex Pharmaceuticals Inc [Member] | 2015 Collaboration Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount in option exercise payments received											$ 30,000,000.0
Number of exclusive license targets | License								3
Vertex Pharmaceuticals Inc [Member] | Beta Globin
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration							48,900,000
Transaction price allocated to remaining performance obligations							$ 23,800,000
ViaCyte [Member] | Research Collaboration Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense																					15,000,000.0
Amount payable under collaboration agreement						$ 15,000,000.0
Number of installments for amount payable | Installment						2
Issuance of common shares | shares				214,512	165,636
Issuance of common shares, value				$ 8,100,000	$ 7,500,000
Key provisions of collaborative agreement description																			The agreement includes certain provisions such that in the event ViaCyte sold shares received from the Company for less than $15.0 million in combined net proceeds, the Company would owe ViaCyte the deficient amount. In the event ViaCyte sold shares received from the Company for greater than $15.0 million in combined net proceeds, ViaCyte would owe the Company the surplus amount.
Net proceeds from issuance of common shares									$ 7,500,000	$ 6,900,000
Deficient amount payable										$ 600,000
Deficient amount paid in cash			$ 600,000
Total consideration paid																					16,200,000
Other (expense) income																					$ (1,200,000)
Collaboration agreement period						6 years
ViaCyte [Member] | Research Collaboration Agreement [Member] | Convertible Promissory Note [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Additional amount payable under certain circumstances						$ 10,000,000.0

[1] Including the following amounts of revenue from a related party, see Note 7

[2] Including the following amounts of research and development from a related party, see Note 7