| Document and Entity Information | 12 Months Ended |
| Dec. 31, 2022 | |
| Cover [Abstract] | |
| Document Type | DEF 14A |
| Amendment Flag | false |
| Entity Registrant Name | REGENXBIO Inc. |
| Entity Central Index Key | 0001590877 |
| Pay vs Performance Disclosure - USD ($) | 12 Months Ended | ||
| Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | |
| Pay vs Performance Disclosure [Table] | |||
| Pay vs Performance [Table Text Block] | Pay Versus Performance The following table provides the total compensation, as reported in the Summary Compensation Table ("SCT"), and the compensation actually paid ("CAP") to our Principal Executive Officer ("PEO") and, on an average basis, our other NEOs ("non-PEO NEOs") for each of the years specified, alongside our total shareholder return ("TSR"), the TSR of our peers, our net income (loss) and a Company-selected financial performance measure of ending cash, cash equivalents and marketable securities. The Company selected ending cash, cash equivalents and marketable securities as the most important financial performance measure used to link compensation to performance in 2022 given its direct influence on our ability to maintain financial strength to achieve corporate objectives. Value of Initial Fixed $100 Investment Based On: Year Summary Compensation Table Total for PEO (1) Compensation Actually Paid to PEO (2)(3) Average Summary Compensation Table Total for Non-PEO NEOs (4) Average Compensation Actually Paid to Non-PEO NEOs (2)(5) Total Shareholder Return of the Company (6) Total Shareholder Return of the Nasdaq Biotechnology Index (7) Net Income (Loss) (8) Ending Cash, Cash Equivalents and Marketable Securities (8) 2022 5,753,713 1,411,167 1,988,042 828,152 55.36 113.65 ( 280,321 ) 565,202 2021 8,622,941 2,648,117 3,207,479 1,658,040 79.81 126.45 127,840 849,346 2020 7,644,087 7,578,195 1,820,462 1,729,261 110.72 126.42 ( 111,250 ) 522,549 (1) Reflects the total compensation of Kenneth T. Mills as reported in the SCT for each of the years presented. (2) Reflects the CAP calculated in accordance with SEC rules. CAP includes unpaid amounts of equity compensation that may be realizable in future periods, and as such, does not fully represent the actual final amount of compensation earned or actually paid during the year. For each year presented, CAP is equal to total compensation reported in the SCT for the year, adjusted as follows: (i) less compensation reported in the SCT related to equity awards which is based on the grant date fair value of equity awards granted during the reporting year, (ii) plus the reporting year-end fair value of equity awards granted during the reporting year that were outstanding and unvested as of the end of the reporting year, (iii) plus the change in fair value from the end of the prior year to the end of the reporting year of equity awards granted in prior years that were outstanding and unvested as of the end of the reporting year, (iv) plus the change in fair value from the end of the prior year to the vesting date for equity awards granted in prior years that vested in the reporting year. None of the equity awards granted to NEOs during the reporting years presented vested in the same reporting year as the grant and there were no forfeitures of equity awards granted to NEOs during any of the reporting years presented. There were no dividends, dividend equivalents or other earnings paid on equity awards granted to NEOs during any of the reporting years presented. The fair values of equity awards used to determine CAP were calculated in accordance with FASB ASC Topic 718 using similar methodologies and assumptions as those used to calculate the grant date fair values of equity awards reported in the SCT . (3) Reflects the CAP to Kenneth T. Mills for each of the years presented using the calculations discussed in footnote 2. The following table presents a reconciliation of total compensation reported in the SCT to CAP for Kenneth T. Mills for each of the years presented. Reconciliation of SCT Total to CAP for Kenneth T. Mills Year SCT Total Grant Date Fair Value of Awards Granted During the Year Included in SCT Total Year-end Fair Value of Equity Awards Granted During the Year Outstanding and Unvested at Year-end Change in Fair Value During the Year of Equity Awards Granted in Prior Years Outstanding and Unvested at Year-end Change in Fair Value from Prior Year-end to Vesting Date of Equity Awards Granted in Prior Years That Vested During the Year CAP 2022 5,753,713 ( 4,734,006 ) 2,706,578 ( 1,673,456 ) ( 641,662 ) 1,411,167 2021 8,622,941 ( 6,900,606 ) 4,273,852 ( 2,264,660 ) ( 1,083,410 ) 2,648,117 2020 7,644,087 ( 6,646,254 ) 7,228,210 67,721 ( 715,569 ) 7,578,195 (4) Reflects the average total compensation of our non-PEO NEOs as reported in the SCT for each of the years presented. For 2022, our non-PEO NEOs were Vit Vasista, Curran Simpson, Stephen Pakola and Olivier Danos. For 2021 and 2020, our non-PEO NEOs were Vit Vasista, Curran Simpson, Olivier Danos, Ph.D. and Patrick J. Christmas. Since the makeup of our non-PEO NEOs may change from year to year, the amounts reported in this table for 2021 and 2020 will not necessarily be equal to the average total compensation of our non-PEO NEOs reported for such years in the SCT of this Proxy Statement. (5) Reflects the average CAP to our non-PEO NEOs for each of the years presented using the calculations discussed in footnote 2. Refer to footnote 4 for the identification of our non-PEO NEOs for each of the years presented. The following table presents a reconciliation of the average total compensation reported in the SCT to CAP for our non-PEO NEOs for each of the years presented. Reconciliation of SCT Total to CAP for Average of Non-PEO NEOs Year SCT Total Grant Date Fair Value of Awards Granted During the Year Included in SCT Total Year-end Fair Value of Equity Awards Granted During the Year Outstanding and Unvested at Year-end Change in Fair Value During the Year of Equity Awards Granted in Prior Years Outstanding and Unvested at Year-end Change in Fair Value from Prior Year-end to Vesting Date of Equity Awards Granted in Prior Years That Vested During the Year CAP 2022 1,988,042 ( 1,300,311 ) 743,295 ( 435,690 ) ( 167,184 ) 828,152 2021 3,207,479 ( 2,174,538 ) 1,346,689 ( 456,963 ) ( 264,627 ) 1,658,040 2020 1,820,462 ( 1,193,864 ) 1,298,401 22,196 ( 217,934 ) 1,729,261 (6) For each reporting year presented, amount represents the cumulative TSR on our common stock between December 31, 2019 and the end of the reporting year, assuming an investment of $100 in our common stock at the closing price on December 31, 2019 and the reinvestment of dividends, if any. Information used for this calculation was obtained from the Nasdaq Stock Market LLC, a financial data provider and a source believed to be reliable. The Nasdaq Stock Market LLC is not responsible for any errors or omissions in such information. (7) For each reporting year presented, amount represents the cumulative TSR of the Nasdaq Biotechnology Index between December 31, 2019 and the end of the reporting year, assuming an investment of $100 in the Nasdaq Biotechnology Index at the closing price on December 31, 2019 and the reinvestment of dividends, if any. Information used for this calculation was obtained from the Nasdaq Stock Market LLC, a financial data provider and a source believed to be reliable. The Nasdaq Stock Market LLC is not responsible for any errors or omissions in such information. (8) As reported in our audited consolidated financial statements accompanying our Annual Report on Form 10-K for the years ended December 31, 2022, 2021 and 2020. | ||
| Company Selected Measure Name | Ending Cash, Cash Equivalents and Marketable Securities | ||
| Named Executive Officers, Footnote [Text Block] | (1) Reflects the total compensation of Kenneth T. Mills as reported in the SCT for each of the years presented. | ||
| PEO Total Compensation Amount | $ 5,753,713 | $ 8,622,941 | $ 7,644,087 |
| PEO Actually Paid Compensation Amount | $ 1,411,167 | 2,648,117 | 7,578,195 |
| Adjustment To PEO Compensation, Footnote [Text Block] | The following table presents a reconciliation of total compensation reported in the SCT to CAP for Kenneth T. Mills for each of the years presented. Reconciliation of SCT Total to CAP for Kenneth T. Mills Year SCT Total Grant Date Fair Value of Awards Granted During the Year Included in SCT Total Year-end Fair Value of Equity Awards Granted During the Year Outstanding and Unvested at Year-end Change in Fair Value During the Year of Equity Awards Granted in Prior Years Outstanding and Unvested at Year-end Change in Fair Value from Prior Year-end to Vesting Date of Equity Awards Granted in Prior Years That Vested During the Year CAP 2022 5,753,713 ( 4,734,006 ) 2,706,578 ( 1,673,456 ) ( 641,662 ) 1,411,167 2021 8,622,941 ( 6,900,606 ) 4,273,852 ( 2,264,660 ) ( 1,083,410 ) 2,648,117 2020 7,644,087 ( 6,646,254 ) 7,228,210 67,721 ( 715,569 ) 7,578,195 | ||
| Non-PEO NEO Average Total Compensation Amount | $ 1,988,042 | 3,207,479 | 1,820,462 |
| Non-PEO NEO Average Compensation Actually Paid Amount | $ 828,152 | 1,658,040 | 1,729,261 |
| Adjustment to Non-PEO NEO Compensation Footnote [Text Block] | The following table presents a reconciliation of the average total compensation reported in the SCT to CAP for our non-PEO NEOs for each of the years presented. Reconciliation of SCT Total to CAP for Average of Non-PEO NEOs Year SCT Total Grant Date Fair Value of Awards Granted During the Year Included in SCT Total Year-end Fair Value of Equity Awards Granted During the Year Outstanding and Unvested at Year-end Change in Fair Value During the Year of Equity Awards Granted in Prior Years Outstanding and Unvested at Year-end Change in Fair Value from Prior Year-end to Vesting Date of Equity Awards Granted in Prior Years That Vested During the Year CAP 2022 1,988,042 ( 1,300,311 ) 743,295 ( 435,690 ) ( 167,184 ) 828,152 2021 3,207,479 ( 2,174,538 ) 1,346,689 ( 456,963 ) ( 264,627 ) 1,658,040 2020 1,820,462 ( 1,193,864 ) 1,298,401 22,196 ( 217,934 ) 1,729,261 | ||
| Equity Valuation Assumption Difference, Footnote [Text Block] | Reflects the CAP calculated in accordance with SEC rules. CAP includes unpaid amounts of equity compensation that may be realizable in future periods, and as such, does not fully represent the actual final amount of compensation earned or actually paid during the year. For each year presented, CAP is equal to total compensation reported in the SCT for the year, adjusted as follows: (i) less compensation reported in the SCT related to equity awards which is based on the grant date fair value of equity awards granted during the reporting year, (ii) plus the reporting year-end fair value of equity awards granted during the reporting year that were outstanding and unvested as of the end of the reporting year, (iii) plus the change in fair value from the end of the prior year to the end of the reporting year of equity awards granted in prior years that were outstanding and unvested as of the end of the reporting year, (iv) plus the change in fair value from the end of the prior year to the vesting date for equity awards granted in prior years that vested in the reporting year. None of the equity awards granted to NEOs during the reporting years presented vested in the same reporting year as the grant and there were no forfeitures of equity awards granted to NEOs during any of the reporting years presented. There were no dividends, dividend equivalents or other earnings paid on equity awards granted to NEOs during any of the reporting years presented. The fair values of equity awards used to determine CAP were calculated in accordance with FASB ASC Topic 718 using similar methodologies and assumptions as those used to calculate the grant date fair values of equity awards reported in the SCT | ||
| Compensation Actually Paid vs. Total Shareholder Return [Text Block] | ion Actually Paid and Total Shareholder Return Relationship Between Compensa tion Actually Paid a nd Net Income (Loss) | ||
| Compensation Actually Paid vs. Net Income [Text Block] | tion Actually Paid a Relationship Between Compensation Actually Paid and Cash, C | ||
| Compensation Actually Paid vs. Company Selected Measure [Text Block] | Relationship Between Compensation Actually Paid and Cash, C ash Equivalents & Marketable Securities | ||
| Tabular List [Table Text Block] | Most Important Financial Performance Measures (Unranked) - Ending balance in cash, cash equivalents and marketable securities - Revenue | ||
| Total Shareholder Return Amount | $ 55.36 | 79.81 | 110.72 |
| Peer Group Total Shareholder Return Amount | 113.65 | 126.45 | 126.42 |
| Net Income (Loss) | $ (280,321,000) | $ 127,840,000 | $ (111,250,000) |
| Company Selected Measure Amount | 565,202,000 | 849,346,000 | 522,549,000 |
| PEO Name | Kenneth T. Mills | Kenneth T. Mills | Kenneth T. Mills |
| Measure [Axis]: 1 | |||
| Pay vs Performance Disclosure [Table] | |||
| Measure Name | Ending balance in cash, cash equivalents and marketable securities | ||
| Measure [Axis]: 2 | |||
| Pay vs Performance Disclosure [Table] | |||
| Measure Name | Revenue | ||
| PEO [Member] | Grant Date Fair Value of Awards Granted During the Year Included in SCT Total [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | $ (4,734,006) | $ (6,900,606) | $ (6,646,254) |
| PEO [Member] | Year-end Fair Value of Equity Awards Granted During the Year Outstanding and Unvested at Year-end [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | 2,706,578 | 4,273,852 | 7,228,210 |
| PEO [Member] | Change in Fair Value During the Year of Equity Awards Granted in Prior Years Outstanding and Unvested at Year-end [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | (1,673,456) | (2,264,660) | 67,721 |
| PEO [Member] | Change in Fair Value from Prior Year-end to Vesting Date of Equity Awards Granted in Prior Years That Vested During the Year [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | (641,662) | (1,083,410) | (715,569) |
| Non-PEO NEO [Member] | Grant Date Fair Value of Awards Granted During the Year Included in SCT Total [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | (1,300,311) | (2,174,538) | (1,193,864) |
| Non-PEO NEO [Member] | Year-end Fair Value of Equity Awards Granted During the Year Outstanding and Unvested at Year-end [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | 743,295 | 1,346,689 | 1,298,401 |
| Non-PEO NEO [Member] | Change in Fair Value During the Year of Equity Awards Granted in Prior Years Outstanding and Unvested at Year-end [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | (435,690) | (456,963) | 22,196 |
| Non-PEO NEO [Member] | Change in Fair Value from Prior Year-end to Vesting Date of Equity Awards Granted in Prior Years That Vested During the Year [Member] | |||
| Pay vs Performance Disclosure [Table] | |||
| Adjustment to Compensation Amount | $ (167,184) | $ (264,627) | $ (217,934) |
