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Share-based compensation - Additional Information (Detail) - USD ($) $ in Millions |  |  | 1 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | ---
Jun. 07, 2018 | Sep. 10, 2004 | Sep. 17, 2014 | Aug. 27, 2008 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |  |
Share based compensation expense |  |  |  |  | $ 0.6 | $ 0.6 |
Issuance of stock option to purchase stock |  |  |  |  | 7,000,000 |  |
Common stock reserved for issuance |  |  |  |  | 6,709,854 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross |  |  |  |  | 307,355 | 0 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value |  |  |  |  | $ 0.0 |  | $ 5.5
Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value, Nonvested |  |  |  |  | 0.0 |  | $ 0.0
Share based Compensation Arrangement By Share based Payment Award Options Exercisable And Vested Intrinsic Value1 |  |  |  |  | $ 0.0 | $ 5.2 |
2004 Stock Incentive Plan |  |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |  |
Common stock reserved for issuance |  | 2,500,000 |  |  |  |  |
Plan expiration date |  | Sep. 10, 2014 |  |  |  |  |
2008 Stock Incentive Plan |  |  |  |  |  |  |
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] |  |  |  |  |  |  |
Common stock reserved for issuance |  |  |  | 3,000,000 |  |  |
Plan expiration date |  |  |  | Aug. 27, 2018 |  |  |
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 |  |  |  |  |  |
Plan expiration date | Jun. 07, 2028 |  |  |  |  |  |
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 Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable and vested. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award percentage from fair market price. + References No definition available. + Details
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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 Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + 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=109197908&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=109197908&loc=d3e5070-113901 + Details
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- Definition Number of common shares reserved for future issuance related to deferred compensation arrangements with individuals. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Date the equity-based award expires, in CCYY-MM-DD format. + 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)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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=109197908&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + 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 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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