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Share-based compensation - Additional Information (Details) - USD ($) |  | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 07, 2018 | Sep. 17, 2014 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |
Share based compensation expense |  |  | $ 900,000 | $ 600,000 |
Issuance of stock option to purchase stock |  |  | 7,000,000 |  |
Common stock reserved for issuance |  |  | 2,717,400 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value |  |  | $ 21,600 |  | $ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value, Nonvested |  |  | 0 |  | $ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value |  |  | $ 11,100 | $ 0 |
Options granted |  |  | 524,800 | 307,355 | 3,874,200
2014 Stock Incentive Plan |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |
Common stock reserved for issuance |  | 3,000,000 |  |  |
Share based compensation arrangement by share based payment award percentage of fair market Person holding more then 10% voting stock |  | 110.00% |  |  |
2018 Stock Incentive Plan |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |
Common stock reserved for issuance | 4,000,000 |  |  |  |
Share based compensation arrangement by share based payment award percentage of fair market Person holding more then 10% voting stock | 110.00% |  |  |  |

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- Definition Intrinsic value of equity-based compensation awards not vested. Stock units + References No definition available. + Details
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- Definition Amount of expense for award under share-based payment arrangement. Excludes amount capitalized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Number of shares authorized for issuance under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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