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Investments - Additional Information (Detail)	6 Months Ended
	Sep. 30, 2014
Minimum [Member]
Investment Securities [Line Items]
Treasury with original maturities period	3 months
Maximum [Member]
Investment Securities [Line Items]
Investment maturity date range end	May 13, 2015
Treasury with original maturities period	1 year

X

- Definition

Original maturity period for held to maturity securities.

+ References

No definition available.

+ Details

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

X

- Details

Name:	ghm_InvestmentSecuritiesLineItems
Namespace Prefix:	ghm_
Data Type:	xbrli:stringItemType
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

Name:	invest_InvestmentMaturityDate1
Namespace Prefix:	invest_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration