Common Stock and Warrants - Summary of Weighted Average Assumptions Used in Estimating Fair Value of Warrants (Details)	Mar. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares	Apr. 24, 2018 $ / shares	Apr. 13, 2018 $ / shares	Apr. 18, 2016 $ / shares
April 2016 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Exercise price					$ 7.50
April 2018 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Exercise price				$ 12.25
Warrants Granted | April 2016 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Stock price	$ 12.65	$ 12.87			5.45
Exercise price	$ 7.50	$ 7.50			$ 7.50
Warrants and Rights Outstanding, Type [Extensible List]	hsdt:BlackScholesOptionPricingModelMember	hsdt:BlackScholesOptionPricingModelMember			hsdt:BlackScholesOptionPricingModelMember
Warrants Granted | April 2018 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Stock price	$ 8.95		$ 10.76	9.85
Exercise price	$ 12.25		$ 12.25	$ 12.25
Warrants and Rights Outstanding, Type [Extensible List]	hsdt:BlackScholesOptionPricingModelMember		hsdt:BlackScholesOptionPricingModelMember	hsdt:BlackScholesOptionPricingModelMember
Warrants | Derivative Financial Instruments Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Stock price	$ 8.95	$ 12.80
Exercise price	$ 10.91	$ 10.89
Warrants and Rights Outstanding, Type [Extensible List]	hsdt:BlackScholesOptionPricingModelMember	hsdt:BlackScholesOptionPricingModelMember
Warrant Term | Warrants Granted | April 2016 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, term	18 days	3 months 18 days			3 years
Warrant Term | Warrants Granted | April 2018 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, term	2 years		3 years	3 years
Warrant Term | Warrants | Derivative Financial Instruments Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, term	1 year 5 months 19 days	1 year 8 months 15 days
Expected Volatility | Warrants Granted | April 2016 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	58.52	83.56			83.83
Expected Volatility | Warrants Granted | April 2018 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	69.14		64.49	64.20
Expected Volatility | Warrants | Derivative Financial Instruments Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	66.13	75.31
Risk-Free Interest Rate | Warrants Granted | April 2016 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	1.68	1.64			0.60
Risk-Free Interest Rate | Warrants Granted | April 2018 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	1.55		2.02	1.99
Risk-Free Interest Rate | Warrants | Derivative Financial Instruments Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	1.59	1.80
Dividend Rate | Warrants Granted | April 2016 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	0.00	0.00			0.00
Dividend Rate | Warrants Granted | April 2018 Offering
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	0.00		0.00	0.00
Dividend Rate | Warrants | Derivative Financial Instruments Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input	0.00	0.00