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Stock-Based Compensation - Assumptions Used Estimate Fair Value of Employee Stock Purchase Plans (Detail) - $ / shares shares in Thousands | 6 Months Ended | 12 Months Ended
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Sep. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |  |  |  |  |  |
Shares purchased for offering period | 2,195 | 2,112 | 1,843 | 3,250 |  |
Fair value per share | $ 2.28 | $ 1.07 | $ 0.96 | $ 0.75 |  |
Assumptions |  |  |  |  |  |
Risk-free interest rate (average) | 1.84% | 2.44% | 2.29% | 1.92% | 2.36% | 2.90%
Weighted average expected life (years) | 6 months | 6 months | 6 months | 6 months | 6 years 2 months 12 days | 6 years 2 months 12 days
Volatility (average) | 86.30% | 51.70% | 54.30% | 81.40% | 78.70% | 73.80%

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- Definition Share based compensation arrangement by share based payment award fair value assumptions fair value per share. + References No definition available. + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + 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)(iv) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Rate of weighted-average expected volatility for award under share-based payment arrangement. + 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)(ii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + 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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