Revenue Arrangements - Narrative (Details)						1 Months Ended	3 Months Ended								12 Months Ended			40 Months Ended
	Dec. 17, 2019 USD ($)	Jun. 05, 2019 USD ($)	Oct. 31, 2017 USD ($) $ / shares shares	Jun. 06, 2017 USD ($) $ / shares shares	Dec. 02, 2016 USD ($) obligation contract target	Dec. 31, 2016 USD ($) shares	Dec. 31, 2019 USD ($) agreement	Sep. 30, 2019 USD ($)	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 ($) agreement shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Mar. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Accumulated deficit							$ (335,753,000)				$ (280,403,000)				$ (335,753,000)	$ (280,403,000)
Long-term contract liability							14,680,000				13,818,000				14,680,000	13,818,000
Short-term contract liability							1,674,000				959,000				$ 1,674,000	959,000
Number of additional targets | target					4
Sale of stock, number of shares issued in transaction (in shares) | shares															86,325,547
Deferred revenue related to agreement							16,354,000				16,903,000				$ 16,354,000	16,903,000
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights																	$ 10,000,000
Remaining performance obligation							9,200,000								9,200,000
Revenue recognized															6,677,000
Adoption of new accounting principle																			$ 1,830,000
Grant and collaboration revenue							6,654,000	$ 0	$ 13,000	$ 10,000	903,000	$ 0	$ 0	$ 0	6,677,000	903,000	$ 207,000
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
License Agreement For Pompe Disease [Member]
Disaggregation of Revenue [Line Items]
Long-term contract liability							5,300,000								5,300,000
Upfront cash payment	$ 7,000,000.0
Initial Up-front cash payment	2,000,000.0
Initial Upfront execution fee payment	5,000,000.0
Short-term contract liability							1,700,000								1,700,000
Sales milestone payments	$ 237,000,000.0
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
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
Sales milestone payments					$ 255,000,000.0
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				$ 14,700,000				$ 9,200,000	14,700,000
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
Accumulated deficit
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle																			1,830,000
Accumulated deficit | ASU 2014-09 | Skolkovo
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle																			1,800,000
Grant Revenue
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue															$ 100,000	$ 0
Adjustments due to ASU 2014-09 | ASU 2014-09
Disaggregation of Revenue [Line Items]
Accumulated deficit																			1,800,000
Long-term contract liability																			$ (1,800,000)