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Income Taxes - Additional Information (Detail) - USD ($) | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2017
Income Tax [Line Items] |  |  |  |
Decrease in valuation allowance against deferred tax assets |  | $ 2,100,000 |  |
Uncertain tax positions |  | $ 0 | $ 0 | $ 0
Federal corporate tax rate | 35.00% | 21.00% |  |
Deductibility percentage of certain capital expenditures |  | 100.00% |  |
Tax benefit from change in corporate tax rates on deferred tax liabilities | $ 61,000,000 |  |  |
State Income Tax [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Investment credits |  | $ 2,000,000 |  |

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- Definition Amount of income tax benefit for remeasurement of deferred tax liability from change in tax rate pursuant to Tax Cuts and Jobs Act. + References No definition available. + Details
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- Definition The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319 + Details
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