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Australia Research and Development Tax Incentive - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2019 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Research and Development Tax Incentive [Line Items] |  |  |  |  |  |
Unrealized foreign currency exchange losses |  |  |  | $ 13,000 | $ 45,000 |
Zafgen Australia Pty Limited [Member] |  |  |  |  |  |
Research and Development Tax Incentive [Line Items] |  |  |  |  |  |
Unrealized foreign currency exchange losses |  |  |  |  | 100,000 |
Zafgen Australia Pty Limited [Member] | Australia [Member] |  |  |  |  |  |
Research and Development Tax Incentive [Line Items] |  |  |  |  |  |
Tax incentive receivable |  | $ 200,000 |  | 200,000 |  | $ 1,500,000
Zafgen Australia Pty Limited [Member] | Maximum [Member] |  |  |  |  |  |
Research and Development Tax Incentive [Line Items] |  |  |  |  |  |
Unrealized foreign currency exchange losses |  | 100,000 | $ 100,000 | 100,000 |  |
Research and Development [Member] | Zafgen Australia Pty Limited [Member] |  |  |  |  |  |
Research and Development Tax Incentive [Line Items] |  |  |  |  |  |
Percentage of refundable tax incentive | 41.00% |  |  |  |  | 43.50%
Reduction to research and development expenses |  |  | $ 300,000 | $ 200,000 | $ 1,300,000 |
Percentage of reduction in research and development costs |  |  |  | 41.00% | 43.50% |
Research and Development [Member] | Zafgen Australia Pty Limited [Member] | Maximum [Member] |  |  |  |  |  |
Research and Development Tax Incentive [Line Items] |  |  |  |  |  |
Reduction to research and development expenses |  | $ 100,000 |  |  |  |

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- Definition Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30690-110894 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30700-110894 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 + Details
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