Revenue Arrangements - Narrative (Details)							1 Months Ended	3 Months Ended		6 Months Ended		40 Months Ended
	Jun. 13, 2020 USD ($)	Dec. 17, 2019 USD ($)	Jun. 05, 2019 USD ($)	Oct. 31, 2017 USD ($) $ / shares shares	Jun. 06, 2017 USD ($) $ / shares shares	Dec. 02, 2016 USD ($) contract obligation target	Dec. 31, 2016 USD ($) shares	Jun. 30, 2020 USD ($) agreement	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) agreement shares	Jun. 30, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 01, 2019 USD ($)	Dec. 31, 2018 $ / shares	Jan. 01, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Short-term contract liability								$ 1,928,000		$ 1,928,000			$ 1,674,000
Long-term contract liability								16,412,000		$ 16,412,000			14,680,000
Number of additional targets | target						4
Sale of stock, number of shares issued in transaction (in shares) | shares										100,847,810
Deferred revenue related to agreement								18,340,000		$ 18,340,000			16,354,000
Remaining performance obligation								18,300,000		18,300,000
Revenue recognized										14,000
Retained earnings (accumulated deficit)								(379,454,000)		(379,454,000)			(335,753,000)
Grant and collaboration revenue								0	$ 13,000	0	$ 23,000
Spark Letter Agreement
Disaggregation of Revenue [Line Items]
Sale common stock				$ 2,500,000
Premium of average daily VWAP (as a percent)				115.00%
Deferred revenue related to agreement				$ 2,500,000
Aggregate purchase price					$ 5,000,000.0
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) | shares				205,254	324,362
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights						$ 2,700,000
Aggregate purchase price per share (in dollar per share) | $ / shares				$ 24.36	$ 15.41
Reimbursement invoices			$ 1,200,000
Spark License Agreement
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement														$ 6,700,000
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Upfront cash payment	$ 2,000,000.0
Short-term contract liability								300,000		300,000
Long-term contract liability								1,700,000		1,700,000
License Agreement For Pompe Disease
Disaggregation of Revenue [Line Items]
Upfront cash payment		$ 7,000,000.0
Sales milestone payments		237,000,000.0
Short-term contract liability								1,700,000		1,700,000
Long-term contract liability								5,300,000		5,300,000
Initial Up-front cash payment		2,000,000.0
Initial Upfront execution fee payment		$ 5,000,000.0
First Acquisition Right
Disaggregation of Revenue [Line Items]
Aggregate purchase price				$ 5,000,000.0
Skolkovo
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement								100,000		100,000						$ 200,000
Payments received												$ 2,000,000.0
Grants from entities, unused grant funds											100,000
Overspent costs											100,000
Second Acquisition Right
Disaggregation of Revenue [Line Items]
Share price (in dollars per share) | $ / shares				$ 24.36
Premium of average daily VWAP (as a percent)				115.00%
Aggregate price of shares Spark will have the right to purchase				$ 5,000,000.0
Spark License Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment						10,000,000.0
Sales milestone payments						255,000,000.0
Aggregate additional payments due within twelve months						$ 5,000,000.0
Term loan number of payments | contract						2
Two payments due within 12 months of contract date						$ 2,500,000
Number of additional targets | target						4
Variable fee the Company is eligible to receive for each additional target option elected						$ 2,000,000.0
Election period to exercise additional targets						3 years
Additional development and regulatory milestone payments						$ 65,000,000.0
Commercialization milestone payments						$ 365,000,000.0
Period after first commercial sale when the Company is eligible to receive royalties						10 years
Period for prior written notice to terminate license						90 days
Sale of stock, number of shares issued in transaction (in shares) | shares							197,238
Sale common stock							$ 5,000,000.0
Share price (in dollars per share) | $ / shares															$ 25.35
Premium of average daily VWAP (as a percent)															115.00%
Deferred revenue related to agreement								$ 9,200,000		$ 9,200,000			$ 9,200,000
Number of agreements combined and evaluated into a single agreement | agreement								2		2
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights						$ 2,700,000
Remaining performance obligation						12,300,000		$ 2,300,000		$ 2,300,000
Performance obligation per each option						1,300,000
Spark License Agreement | MIT
Disaggregation of Revenue [Line Items]
License and option agreement, additional sub-license payment received liability										2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement										400,000
Spark License Agreement | Consideration for Promises
Disaggregation of Revenue [Line Items]
Upfront cash payment						15,000,000.0
Spark License Agreement | Development Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments						35,000,000.0
Spark License Agreement | Regulatory Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments						30,000,000.0
Spark License Agreement | Commercial Milestones
Disaggregation of Revenue [Line Items]
Commercialization milestone payments						$ 110,000,000.0
Spark License Agreement | License and Supply Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation						5
Remaining performance obligation						$ 7,100,000
Spark License Agreement | Discount on Option Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation						4
Remaining performance obligation						$ 5,200,000
ASU 2014-09 | Skolkovo | Cumulative Effect, Period of Adoption, Adjustment [Member]
Disaggregation of Revenue [Line Items]
Retained earnings (accumulated deficit)																$ 1,800,000
Grant Revenue
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue										$ 100,000	$ 0