Quarterly report pursuant to Section 13 or 15(d)

Intangible Assets - Summary of Intangible Assets (Detail)

v2.4.1.9
Intangible Assets - Summary of Intangible Assets (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Mar. 31, 2014
Schedule Of Intangible Assets [Line Items]    
Finite-Lived Intangible Assets, Gross Carrying Amount $ 2,870us-gaap_FiniteLivedIntangibleAssetsGross $ 2,870us-gaap_FiniteLivedIntangibleAssetsGross
Finite-Lived Intangible Assets, Accumulated Amortization 897us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 762us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Finite-Lived Intangible Assets, Net Carrying Amount 1,973us-gaap_FiniteLivedIntangibleAssetsNet 2,108us-gaap_FiniteLivedIntangibleAssetsNet
Indefinite Lived Intangible Assets, Net Carrying Amount 12,800us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill 12,800us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
Backlog [Member]    
Schedule Of Intangible Assets [Line Items]    
Finite-Lived Intangible Assets, Gross Carrying Amount 170us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OrderOrProductionBacklogMember
170us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OrderOrProductionBacklogMember
Finite-Lived Intangible Assets, Accumulated Amortization 170us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OrderOrProductionBacklogMember
170us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OrderOrProductionBacklogMember
Finite-Lived Intangible Assets, Net Carrying Amount 0us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OrderOrProductionBacklogMember
0us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OrderOrProductionBacklogMember
Customer Relationships [Member]    
Schedule Of Intangible Assets [Line Items]    
Finite-Lived Intangible Assets, Gross Carrying Amount 2,700us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
2,700us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Finite-Lived Intangible Assets, Accumulated Amortization 727us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
592us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Finite-Lived Intangible Assets, Net Carrying Amount 1,973us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
2,108us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Permits [Member]    
Schedule Of Intangible Assets [Line Items]    
Indefinite Lived Intangible Assets, Net Carrying Amount 10,300us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
10,300us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
Tradename [Member]    
Schedule Of Intangible Assets [Line Items]    
Indefinite Lived Intangible Assets, Net Carrying Amount $ 2,500us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
$ 2,500us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember