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Intangible Assets - Summary of Intangible Assets (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | May 31, 2014 | Mar. 31, 2014
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Schedule Of Intangible Assets [Line Items] |  |  |
Indefinite Lived Intangible Assets, Gross Carrying Amount | $ 12,800 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill | $ 12,800 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill |
Indefinite Lived Intangible Assets, Accumulated Amortization | 0 ghm_IndefiniteLivedIntangiblesAccumulatedAmortization | 0 ghm_IndefiniteLivedIntangiblesAccumulatedAmortization |
Indefinite Lived Intangible Assets, Net Carrying Amount | 12,800 us-gaap_IntangibleAssetsNetExcludingGoodwill | 12,800 us-gaap_IntangibleAssetsNetExcludingGoodwill |
Permits [Member] |  |  |
Schedule Of Intangible Assets [Line Items] |  |  |
Indefinite Lived Intangible Assets, Gross Carrying Amount | 10,300 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = ghm_NuclearPermitsMember | 10,300 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = ghm_NuclearPermitsMember |
Indefinite Lived Intangible Assets, Accumulated Amortization | 0 ghm_IndefiniteLivedIntangiblesAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = ghm_NuclearPermitsMember | 0 ghm_IndefiniteLivedIntangiblesAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = ghm_NuclearPermitsMember |
Indefinite Lived Intangible Assets, Net Carrying Amount | 10,300 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = ghm_NuclearPermitsMember | 10,300 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = ghm_NuclearPermitsMember |
Tradename [Member] |  |  |
Schedule Of Intangible Assets [Line Items] |  |  |
Indefinite Lived Intangible Assets, Gross Carrying Amount | 2,500 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember | 2,500 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
Indefinite Lived Intangible Assets, Accumulated Amortization | 0 ghm_IndefiniteLivedIntangiblesAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember | 0 ghm_IndefiniteLivedIntangiblesAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
Indefinite Lived Intangible Assets, Net Carrying Amount | 2,500 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember | 2,500 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
Customer Relationships [Member] |  |  |
Schedule Of Intangible Assets [Line Items] |  |  |
Finite-Lived Intangible Assets, Gross Carrying Amount | 2,700 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |  | 2,700 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Finite-Lived Intangible Assets, Accumulated Amortization | 772 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |  | 592 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Finite-Lived Intangible Assets, Net Carrying Amount | $ 1,928 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |  | $ 2,108 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember

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- Definition Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph ((a)(1),(b)) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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