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Share Based Payments - Summary of Company's Restricted Stock Unit 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] |  |
Number of RSUs withheld for taxes | 259 | 723

X
- Definition Number of restricted stock units withheld for taxes. + 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