Significant Contracts - Additional Information (Detail)					1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
		Feb. 12, 2016 USD ($)	Dec. 19, 2015 USD ($) $ / shares	Oct. 26, 2015 USD ($) Milestone	May 31, 2018 USD ($)	Oct. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	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]						$ 1,088,000	$ 3,582,000	$ 2,446,000	$ 6,285,000
Research and development expense	[2]						25,633,000	17,120,000	45,152,000	31,925,000
Non-current deferred revenue							57,832,000		$ 57,832,000		$ 56,928,000
Date of joint venture agreement									Dec. 19, 2015
Proceeds from issuance of common shares in secondary offering, net of issuance						$ 35,000,000			$ 122,597,000
Standalone selling price allocation of arrangement consideration paid							106,300,000		106,300,000
Other income							155,000	(161,000)	29,000	(167,000)		$ 79,100,000
Equity method investment												36,500,000
Unrealized equity method losses							(1,153,000)	(505,000)	(2,244,000)	(951,000)		$ (36,500,000)
Stock-based compensation expense					$ 2,200,000		9,477,000	4,231,000	16,150,000	7,977,000
ASC 606 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-current deferred revenue													$ 57,974,000
ASC 606 [Member] | ASC 605 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue							994,000		2,251,000
Non-current deferred revenue							56,981,000		56,981,000
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
Series B Preferred Shares [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Debt instrument, conversion price | $ / shares			$ 13.43
Bayer Convertible Loans [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Gross proceeds from convertible debt			$ 35,000,000
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							1,200,000	1,400,000	2,400,000	2,500,000
Reimbursements from research and license agreements							600,000	900,000	900,000	1,500,000
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							1,200,000	500,000	2,200,000	1,000,000
Unrecognized equity method losses in excess of Company's interest									33,000,000		21,200,000
Operating expenses of joint venture							13,500,000	9,000,000	13,500,000	14,600,000
Net loss of joint venture							13,500,000	9,000,000	25,800,000	14,600,000
Research and Development Expenses [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock-based compensation expense							5,167,000	1,946,000	8,083,000	3,676,000
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								1,500,000		2,600,000
License and Service [Member] | Casebia Therapeutics LLP [Member] | ASC 606 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue							900,000		2,000,000
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							200,000	3,300,000	400,000	5,900,000
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							200,000	$ 2,100,000	400,000	$ 3,600,000
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							8,900,000		$ 16,300,000
Reimbursements from research and license agreements							$ 3,700,000		$ 6,900,000
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

[1] Including the following revenue from a related party, see Note 10:

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