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Common Stock and Warrants - Summary of Warrants Accounted for as Liabilities and Recorded as Derivative Financial Instruments (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)
Fair Value Disclosures [Abstract]
Fair value of warrants at beginning of period	$ 5
Change in fair value of warrants during the period	(4)
Fair value of warrants at end of period	$ 1

X

- Definition

Fair value of warrants liability.

+ References

No definition available.

+ Details

Name:	hsdt_FairValueOfWarrantsLiability
Namespace Prefix:	hsdt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of expense (income) related to adjustment to fair value of warrant liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section 25
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=109262497&loc=d3e20148-110875

+ Details

Name:	us-gaap_FairValueAdjustmentOfWarrants
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

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