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Income Taxes - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2013
Income Taxes [Line Items] |  |  |  |  |
Liability unrecognized tax benefits | $ 0 |  | $ 0 |  | $ 134
Reduction in Liability for unrecognized tax benefits |  |  | 134 |  |
Interest uncertain tax positions | (15) | 2 | (11) | (323) |
Interest uncertain tax positions reversed |  |  |  | 387 |
Penalties related to uncertain tax positions | $ 0 | $ 0 | $ 0 | $ 0 |
Maximum [Member] | Internal Revenue Service (IRS) [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2012 |  |
Maximum [Member] | Federal Tax Jurisdictions [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2013 |  |
Maximum [Member] | State Tax Jurisdictions [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2013 |  |
Maximum [Member] | International Tax Jurisdictions [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2013 |  |
Minimum [Member] | Internal Revenue Service (IRS) [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2011 |  |
Minimum [Member] | Federal Tax Jurisdictions [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2011 |  |
Minimum [Member] | State Tax Jurisdictions [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2009 |  |
Minimum [Member] | International Tax Jurisdictions [Member] |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Federal and State tax jurisdictions for tax period |  |  | 2011 |  |

X
- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The gross amount of unrecognized tax benefit pertaining to uncertain tax position for the research and development tax credit taken in tax returns as of the balance sheet date. + References No definition available. + Details
Name: | ghm_UnrecognizedTaxBenefitOfResearchAndDevelopmentTaxCredit
Namespace Prefix: | ghm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This element represents the reversal of accrued interest for an overpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity + References No definition available. + Details
Name: | ghm_UnrecognizedTaxBenefitsInterestOnIncomeTaxesReversal
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Tax year being audited in the income tax examination, in CCYY format. + References No definition available. + Details
Name: | us-gaap_IncomeTaxExaminationYearUnderExamination
Namespace Prefix: | us-gaap_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tax year that remains open to examination under enacted tax laws, in CCYY format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
Name: | us-gaap_OpenTaxYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of expense for penalties related to a tax position claimed or expected to be claimed in the tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of interest expense for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition The net amount of all increases and decreases in unrecognized tax benefits for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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