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v2.4.1.9

Investments - Additional Information (Detail) | 9 Months Ended
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Dec. 31, 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 | Nov. 17, 2015
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

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- 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

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |