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Stock-Based Compensation - Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model (Detail) - Options Issued and Outstanding - $ / shares | 3 Months Ended | 12 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Employees |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |
Expected term (in years) |  | 6 years 1 month 6 days |  |  |
Expected volatility, minimum |  | 93.70% | 92.00% | 75.90% | 74.40%
Expected volatility, maximum |  | 94.80% | 104.90% | 90.60% | 74.50%
Risk-free interest rate, minimum |  | 2.50% | 1.40% | 2.40% | 1.90%
Risk-free interest rate, maximum |  | 2.60% | 2.60% | 3.10% | 2.20%
Expected dividend yield |  | 0.00% | 0.00% | 0.00% | 0.00%
Non-employees |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |
Expected volatility, minimum | 99.60% | 91.30% | 89.70% | 78.00% | 74.60%
Expected volatility, maximum | 101.70% | 94.90% | 99.50% | 91.20% | 77.00%
Risk-free interest rate, minimum | 0.60% | 2.40% | 1.70% | 2.70% | 2.30%
Risk-free interest rate, maximum | 0.90% | 2.60% | 2.80% | 3.10% | 2.40%
Expected dividend yield | 0.00% | 0.00% | 0.00% | 0.00% | 0.00%
Minimum | Employees |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |
Expected term (in years) |  |  | 5 years 6 months | 5 years 9 months 18 days | 5 years 10 months 24 days
Fair value of common stock |  | $ 8.53 | $ 1.27 | $ 8.57 | $ 0.79
Minimum | Non-employees |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |
Expected term (in years) | 7 years 2 months 12 days | 8 years 2 months 12 days | 7 years 4 months 24 days | 8 years 4 months 24 days | 9 years 4 months 24 days
Fair value of common stock | $ 0.96 | $ 6.82 | $ 1.23 | $ 8.62 | $ 0.79
Maximum | Employees |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |
Expected term (in years) |  |  | 6 years 1 month 6 days | 6 years 2 months 12 days | 6 years 2 months 12 days
Fair value of common stock |  | $ 8.9 | $ 10.66 | $ 15.45 | $ 9.33
Maximum | Non-employees |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |
Expected term (in years) | 9 years | 10 years | 10 years | 10 years | 10 years
Fair value of common stock | $ 1.07 | $ 8.61 | $ 10.26 | $ 15.45 | $ 10.28

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- Definition Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Fair Value Of Common Stock + References No definition available. + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References No definition available. + Details
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- Definition The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References No definition available. + Details
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- Definition The maximum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition The minimum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 + Details
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