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Investments - Additional Information (Detail)	3 Months Ended
	Jun. 30, 2018
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	Dec. 27, 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

The maturity date of the investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

+ Details

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Namespace Prefix:	invest_
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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
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X

- Details

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