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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
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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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