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Other Income - Additional Information (Detail) $ in Thousands | 12 Months Ended
--- | ---
Mar. 31, 2016 USD ($)
Other Income And Expenses [Abstract] |
Other income | $ 1,789

X
- Definition Cancellation fee income earned from customers due to the cancellation of orders by customers. + References No definition available. + Details
Name: | ghm_CancellationFeeIncome
Namespace Prefix: | ghm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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