Significant Contracts - Additional Information (Detail)								1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
		Nov. 07, 2018 USD ($)	Sep. 24, 2018 USD ($) shares	Sep. 17, 2018 USD ($) Installment	Feb. 12, 2016 USD ($)	Dec. 19, 2015 USD ($) $ / shares	Oct. 26, 2015 USD ($) Milestone	Oct. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue	[1]								$ 563,000	$ 2,387,000	$ 3,009,000	$ 8,672,000
Research and development expense	[2]								39,820,000	17,845,000	84,972,000	49,770,000
Non-current deferred revenue									57,806,000		$ 57,806,000		$ 56,928,000
Date of joint venture agreement											Dec. 19, 2015
Net proceeds from issuance of common shares								$ 35,000,000			$ 309,019,000
Standalone selling price allocation of arrangement consideration paid									106,300,000		106,300,000
Other income									(130,000)	(71,000)	(101,000)	(238,000)		$ 79,100,000
Equity method investment														36,500,000
Unrealized equity method losses									(1,012,000)	(359,000)	(3,256,000)	(1,310,000)		$ (36,500,000)
Stock-based compensation expense									9,067,000	5,227,000	25,216,000	13,204,000
Total liabilities									101,589,000		101,589,000		83,514,000
ASC 606 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-current deferred revenue															$ 57,974,000
Total liabilities															$ 84,662,000
ASC 606 [Member] | ASC 605 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue									506,000		2,757,000
Non-current deferred revenue									57,012,000		57,012,000
Total liabilities									100,693,000		100,693,000
Licensing Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Standalone selling price									71,400,000		71,400,000
License and Patent Holder Consent [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Standalone selling price allocation of arrangement consideration paid									79,100,000		79,100,000
Research and Development Services [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Standalone selling price									6,300,000		6,300,000
Common Stock [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Shares issued and sold | shares								2,500,000
Shares issued, price per share | $ / shares								$ 14.00
Bayer Convertible Loans [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Gross proceeds from convertible debt						$ 35,000,000
Debt instrument converted to Series B Preferred Shares, conversion price | $ / shares						$ 13.43
Fair value of convertible loan									24,500,000		24,500,000
Standalone selling price allocation of arrangement consideration paid									27,200,000		27,200,000
Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue											112,600,000
Research and development expense									400,000	1,200,000	2,800,000	3,700,000
Reimbursements from research and license agreements									0	400,000	900,000	2,100,000
Non-current deferred revenue									$ 0		$ 0		100,000
Date of formation of joint venture entity											Feb. 12, 2016
Equity method investment, ownership percentage					50.00%				50.00%		50.00%
Cash contribution					$ 100,000
Other payments due to company from joint venture entity					0
Cash contribution											$ 100,000
Equity interest in the Joint Venture									$ 36,400,000		36,400,000
Convertible Loan									35,000,000		35,000,000
Estimated revenue related to research and development services											6,300,000
Equity method investment									0		0		0
Stock-based compensation expense									1,000,000	400,000	3,300,000	1,300,000
Unrecognized equity method losses in excess of Company's interest											38,800,000		21,200,000
Operating expenses of joint venture									12,800,000	10,200,000	39,100,000	24,700,000
Net loss of joint venture									12,600,000	10,200,000	38,400,000	24,700,000
Research and Development Expenses [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock-based compensation expense									3,999,000	2,420,000	12,081,000	6,095,000
License and Service [Member] | Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue					35,000,000
License and Service [Member] | Casebia Therapeutics LLP [Member] | ASC 605 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue											1,200,000	3,900,000
License and Service [Member] | Casebia Therapeutics LLP [Member] | ASC 606 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue									400,000	2,400,000
Royalty [Member] | Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue					$ 0
Technology Service [Member] | Casebia Therapeutics LLP [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue											34,900,000
Non-Exclusive Research License [Member] | Market-based Approach [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for collaboration agreement							$ 1,000,000
Vertex Pharmaceuticals Inc [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Nonrefundable upfront payment received							75,000,000
Potential milestone receivable							420,000,000
Non-exclusive research revenue							19,300,000
Estimated standalone selling price for collaboration agreement							118,600,000
Revenue, Remaining Performance Obligation							82,000,000		57,900,000		57,900,000
Clinical milestone payment payable							10,000,000
Research and development expense									100,000	2,100,000	500,000	8,000,000
Non-current deferred revenue									57,800,000		57,800,000		$ 56,800,000
Vertex Pharmaceuticals Inc [Member] | Research and Development Expenses [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Variable consideration received							19,300,000
Vertex Pharmaceuticals Inc [Member] | 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] | 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] | 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] | License and Service [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue									100,000	$ 1,200,000	500,000	$ 4,800,000
Vertex Pharmaceuticals Inc [Member] | Joint Development Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Nonrefundable upfront payment received							7,000,000
Up-front payment received											$ 7,000,000
Agreement description											In connection with entering into the JDA, the Company received a $7.0 million up-front payment from Vertex and is eligible for a one-time low seven-digit milestone payment upon the dosing of the second patient in a clinical trial with the initial product candidate. The net profits and net losses, as applicable, incurred under the JDA will be shared equally between the Company and Vertex.
Research and development expense									8,700,000		$ 24,900,000
Reimbursements from research and license agreements									3,700,000		$ 10,400,000
Vertex Pharmaceuticals Inc [Member] | Beta-Globin [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated standalone selling price for single collaboration							48,900,000
Vertex Pharmaceuticals Inc [Member] | Non-Exclusive Research License [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							500,000
Vertex Pharmaceuticals Inc [Member] | Material Right to Discounts for Exclusive Licenses [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							57,700,000
Vertex Pharmaceuticals Inc [Member] | Material Right to Discounts for Exclusive Licenses [Member] | Unit One [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							22,200,000
Vertex Pharmaceuticals Inc [Member] | Material Right to Discounts for Exclusive Licenses [Member] | Unit Two [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							18,700,000
Vertex Pharmaceuticals Inc [Member] | Material Right to Discounts for Exclusive Licenses [Member] | Unit Three [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							8,400,000
Vertex Pharmaceuticals Inc [Member] | Material Right to Discounts for Exclusive Licenses [Member] | Unit Four [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							8,400,000
Vertex Pharmaceuticals Inc [Member] | Co-exclusive Development and Commercialization Licenses and Research License [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue, Remaining Performance Obligation							$ 23,800,000
Vertex Pharmaceuticals Inc [Member] | Clinical Development And Regulatory Milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of developmental milestone events | Milestone							9
Vertex Pharmaceuticals Inc [Member] | Clinical Development Milestones [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition, milestone method, revenue recognized							$ 90,000,000
Vertex Pharmaceuticals Inc [Member] | Regulatory Approval Milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition, milestone method, revenue recognized							$ 235,000,000
Vertex Pharmaceuticals Inc [Member] | Commercial Milestones [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of developmental milestone events | Milestone							2
Revenue recognition, milestone method, revenue recognized							$ 75,000,000
Vertex Pharmaceuticals Inc [Member] | Investigational New Drug Application ("IND") [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition, milestone method, revenue recognized							10,000,000
Vertex Pharmaceuticals Inc [Member] | Developmental Milestone Events [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition, milestone method, revenue recognized							420,000,000
Vertex Pharmaceuticals Inc [Member] | Commercial Milestone Event One [Member] | Minimum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue							500,000,000
Vertex Pharmaceuticals Inc [Member] | Commercial Milestone Event Two [Member] | Minimum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue							$ 1,000,000,000
ViaCyte [Member] | Research Collaboration Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Shares issued and sold | shares			165,636
Net proceeds from issuance of common shares			$ 6,900,000
Amount payable under collaboration agreement				$ 15,000,000
Number of installments for amount payable | Installment				2
Issuance of common shares, value			$ 7,500,000
Collaboration agreement period				6 years
Key provisions of collaborative agreement description											The agreement includes certain provisions such that in the event ViaCyte sells shares received from the Company for less than $15.0 million in combined net proceeds, the Company will pay ViaCyte the deficient amount. In the event ViaCyte sells shares received from the Company for greater than $15.0 million in combined net proceeds, ViaCyte will pay the Company the surplus amount.
Derivative liability, fair value									100,000		$ 100,000
Change in fair value											100,000
ViaCyte [Member] | Research Collaboration Agreement [Member] | Accrued Expenses [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total liabilities									$ 7,600,000		$ 7,600,000
ViaCyte [Member] | Research Collaboration Agreement [Member] | Subsequent Event
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration agreement fair value of remaining amount payable, either cash or shares		$ 8,100,000
Loss on change in fair value derivative		$ (500,000)
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
ViaCyte [Member] | Research Collaboration Agreement [Member] | Research and Development Expenses [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payment for intellectual property				$ 15,000,000

[1] Including the following revenue from a related party, see Notes 6 & 10:

[2] Including the following research and development expense with a related party, see Notes 6 & 10: