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Investments - Additional Information (Detail)	3 Months Ended
	Jun. 30, 2017
Minimum [Member]
Schedule of Held-to-maturity Securities [Line Items]
Treasury with original maturities period	3 months
Maximum [Member]
Schedule of Held-to-maturity Securities [Line Items]
Investment maturity date range end	May 15, 2018
Treasury with original maturities period	1 year

X

- Definition

Original maturity period for held to maturity securities.

+ References

No definition available.

+ Details

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Namespace Prefix:	ghm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Date of maturity of a debt security categorized as held-to-maturity, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
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Period Type: