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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, 2017 | Mar. 31, 2017 | Sep. 30, 2016 | Mar. 31, 2016 | Sep. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |  |  |  |  |  |  |  |
Shares purchased for offering period | 2,000 | 2,002 | 3,536 | 3,498 | 3,113 | 2,804 |  |
Fair value per share | $ 0.35 | $ 0.72 | $ 0.77 | $ 1.24 | $ 2.39 | $ 1.72 |  |
Assumptions |  |  |  |  |  |  |  |
Risk-free interest rate (average) | 0.89% | 0.47% | 0.47% | 0.18% | 0.11% | 0.40% | 1.10% | 1.10%
Weighted average expected life (years) | 6 months | 6 months | 6 months | 6 months | 6 months | 6 months | 4 years 9 months 18 days | 4 years
Volatility (average) | 64.20% | 52.60% | 40.40% | 66.10% | 103.80% | 109.10% | 74.30% | 83.50%

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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://www.xbrl.org/2003/role/presentationRef -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=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average expected volatility rate of share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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=96867065&loc=d3e5070-113901 + Details
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- Definition Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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=27013229&loc=d3e301413-122809 + Details
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- Definition Number of shares issued during the period as a result of an employee stock purchase plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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