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Income Taxes - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014
Income Taxes [Line Items] |  |
Valuation allowance | 952 us-gaap_DeferredTaxAssetsValuationAllowance | $ 738 us-gaap_DeferredTaxAssetsValuationAllowance
Liability unrecognized tax benefits | 0 us-gaap_UnrecognizedTaxBenefits | 0 us-gaap_UnrecognizedTaxBenefits
Latest Tax Year [Member] | Federal Tax Jurisdictions [Member] |  |
Income Taxes [Line Items] |  |
Open tax year | 2014 |
Latest Tax Year [Member] | State Tax Jurisdictions [Member] |  |
Income Taxes [Line Items] |  |
Open tax year | 2014 |
Latest Tax Year [Member] | International Tax Jurisdictions [Member] |  |
Income Taxes [Line Items] |  |
Open tax year | 2014 |
Earliest Tax Year [Member] | Federal Tax Jurisdictions [Member] |  |
Income Taxes [Line Items] |  |
Open tax year | 2013 |
Earliest Tax Year [Member] | State Tax Jurisdictions [Member] |  |
Income Taxes [Line Items] |  |
Open tax year | 2010 |
Earliest Tax Year [Member] | International Tax Jurisdictions [Member] |  |
Income Taxes [Line Items] |  |
Open tax year | 2012 |
Investment Tax Credit Carryforward [Member] |  |
Income Taxes [Line Items] |  |
Deferred income taxes include the impact of state investment tax credits | 344 ghm_DeferredTaxAssetsTaxDeferredExpenseStateInvestmentTaxCreditsThatExpire / us-gaap_TaxCreditCarryforwardAxis = us-gaap_InvestmentCreditMember |
State investment tax credits with an unlimited carryforward period | 608 ghm_DeferredTaxAssetsTaxDeferredExpenseStateInvestmentTaxCreditsWithUnlimitedCarryforwardPeriod / us-gaap_TaxCreditCarryforwardAxis = us-gaap_InvestmentCreditMember |
Investment Tax Credit Carryforward [Member] | Minimum [Member] |  |
Income Taxes [Line Items] |  |
Expiration date of state investment tax credits | Mar. 31, 2015 |
Investment Tax Credit Carryforward [Member] | Maximum [Member] |  |
Income Taxes [Line Items] |  |
Expiration date of state investment tax credits | Mar. 31, 2029 |

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- Definition The amount of deferred income tax assets pertaining to state investment tax credits that have expiration dates. + References No definition available. + Details
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- Definition The amount of deferred income tax assets pertaining to state investment tax credits that have an unlimited carryforward period. + References No definition available. + Details
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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
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- 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
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- Definition Expiration date of the tax credit carryforward, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: 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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