Share Based Payments - Summary of Company's Restricted Stock Unit Activity (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2017 shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of RSUs withheld for taxes	723

Number of restricted stock units withheld for taxes.

No definition available.

Name:	hsdt_NumberOfRestrictedStockUnitsWithheldForTaxes
Namespace Prefix:	hsdt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration