Product Warranty Liability - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Guarantees [Abstract]
Expense (income) for product warranties	$ 78	$ (54)	$ (82)	$ 111

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.

No definition available.

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