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Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Mar. 31, 2018
Schedule Of Intangible Assets [Line Items] |  |
Intangibles not subject to amortization, Gross Carrying Amount | $ 12,800 | $ 12,800
Intangibles not subject to amortization, Impairment Loss | 9,100 | 9,100
Intangibles not subject to amortization, Net Carrying Amount | 3,700 | 3,700
Permits [Member] |  |
Schedule Of Intangible Assets [Line Items] |  |
Intangibles not subject to amortization, Gross Carrying Amount | 10,300 | 10,300
Intangibles not subject to amortization, Impairment Loss | 8,600 | 8,600
Intangibles not subject to amortization, Net Carrying Amount | 1,700 | 1,700
Tradename [Member] |  |
Schedule Of Intangible Assets [Line Items] |  |
Intangibles not subject to amortization, Gross Carrying Amount | 2,500 | 2,500
Intangibles not subject to amortization, Impairment Loss | 500 | 500
Intangibles not subject to amortization, Net Carrying Amount | 2,000 | 2,000
Customer Relationships [Member] |  |
Schedule Of Intangible Assets [Line Items] |  |
Intangibles subject to amortization, Gross Carrying Amount | 2,700 | 2,700
Intangibles subject to amortization, Accumulated Amortization | 1,402 | 1,312
Intangibles subject to amortization, Net Carrying Amount | $ 1,298 | $ 1,388

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