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Share Based Payments - Summary of Stock Option Activity (Parenthetical) (Details) - shares | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |  |
Stock options exercised on cashless basis | 8,500 | 20,000
Number of shares withheld | 3,280 | 14,095

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- Definition Number of common stock shares withheld. + References No definition available. + Details
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Data Type: | xbrli:sharesItemType
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Period Type: | duration

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- Definition Stock issued during period shares stock options exercised on cashless basis. + References No definition available. + Details
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Namespace Prefix: | hsdt_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration