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Business Segments - Additional Information (Detail) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) segment MW | Sep. 30, 2017 USD ($) | Mar. 31, 2018 segment
Segment Reporting Information [Line Items] |  |  |  |  |
Number of reportable business segments | segment |  |  | 2 |  | 2
Stock-based compensation expense | $ 825 | $ 478 | $ 1,610 | $ 1,232 |
Restructuring charges (less than $0.1 million) | 93 | (12) | 403 | 1,328 |
Gain on fair value of contingent consideration | 0 | 201 | 0 | 201 |
Unallocated corporate expenses |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Stock-based compensation expense | 800 | 500 | 1,600 | 1,200 |
Restructuring charges (less than $0.1 million) | $ 100 | $ 100 | $ 400 | $ 1,300 |
Minimum |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Megawatts of drive trains and power ratings | MW |  |  | 2 |  |

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