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Share Based Payments - Summary of Company's Restricted Stock Unit Activity (Parenthetical) (Details) - shares	12 Months Ended
	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

X

- References

No definition available.

+ Details

Name:	us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration