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Product Warranty Liability - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Guarantees [Abstract]
Expense (income) for product warranties	$ (19)	$ 27

X

- Definition

The (income) expense (included) charged (in) against earnings for the period pertaining to standard and extended warranties on the entity's goods and services granted to customers.

+ References

No definition available.

+ Details

Name:	ghm_ProductWarrantyIncomeExpense
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_GuaranteesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration