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Common Stock and Warrants - Summary of Weighted Average Assumptions Used in Estimating Fair Value of Warrants (Details)	Dec. 31, 2018 $ / shares	Apr. 24, 2018 $ / shares	Apr. 13, 2018 $ / shares	Apr. 01, 2018 $ / shares	Apr. 01, 2018 $ / shares	Dec. 31, 2017 $ / shares	Dec. 29, 2017 $ / shares	Dec. 28, 2017 $ / shares	Dec. 22, 2017 $ / 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	$ 12.80				$ 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					hsdt:BlackScholesOptionPricingModelMember
Warrants Granted | December 2017 Financing
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Stock price				$ 10.11			$ 12.32	$ 12.45	$ 10.60
Exercise price				$ 12.25			$ 12.25	$ 12.25	$ 12.25
Warrants and Rights Outstanding, Type [Extensible List]				hsdt:BlackScholesOptionPricingModelMember	hsdt:BlackScholesOptionPricingModelMember		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	$ 12.80	$ 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	$ 12.80					$ 12.32
Exercise price	$ 10.89					$ 10.25
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	3 months 18 days			1 year 18 days	1 year 18 days					3 years
Warrant Term | Warrants Granted | December 2017 Financing
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, term				2 years 8 months 12 days	2 years 8 months 12 days		3 years	3 years	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 months 10 days	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 8 months 15 days					1 year 10 months 28 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	83.56			71.13	71.13					83.83
Expected Volatility | Warrants Granted | December 2017 Financing
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input				65.40	65.40		60.24	60.24	60.24
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	71.99	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	75.31					62.20
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	1.64			1.60	1.60					0.60
Risk-Free Interest Rate | Warrants Granted | December 2017 Financing
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Weighted average assumptions, measurement input				2.39	2.39		1.98	2.00	2.01
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	1.86	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	1.80					1.83
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					0.00
Dividend Rate | Warrants Granted | December 2017 Financing
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	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

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