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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 |  |  |  |  |

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