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Intangible Assets (Tables) | 12 Months Ended
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Mar. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract] |
Summary of Intangible Assets | Intangible assets are comprised of the following:
|  | Gross Carrying Amount |  |  | Accumulated Amortization |  |  | Impairment Loss |  |  | Net Carrying Amount |
At March 31, 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles subject to amortization: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Customer relationships |  | $ | 2,700 |  |  | $ | 1,312 |  |  | $ | — |  |  | $ | 1,388 |
Intangibles not subject to amortization: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permits |  | $ | 10,300 |  |  | $ | — |  |  | $ | 8,600 |  |  | $ | 1,700 |
Tradename |  |  | 2,500 |  |  |  | — |  |  |  | 500 |  |  |  | 2,000 |
|  | $ | 12,800 |  |  | $ | — |  |  | $ | 9,100 |  |  | $ | 3,700 |
At March 31, 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles subject to amortization: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Customer relationships |  | $ | 2,700 |  |  | $ | 1,132 |  |  | $ | — |  |  | $ | 1,568 |
Intangibles not subject to amortization: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permits |  | $ | 10,300 |  |  | $ | — |  |  | $ | — |  |  | $ | 10,300 |
Tradename |  |  | 2,500 |  |  |  | — |  |  |  | — |  |  |  | 2,500 |
|  | $ | 12,800 |  |  | $ | — |  |  | $ | — |  |  | $ | 12,800 |

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- Definition Tabular disclosure of assets, excluding financial assets and goodwill, lacking physical substance with a finite and indefinite life, by major class. + References No definition available. + Details
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Namespace Prefix: | ghm_
Data Type: | nonnum:textBlockItemType
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Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_GoodwillAndIntangibleAssetsDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration