Annual report pursuant to Section 13 and 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
Mar. 31, 2015
May 31, 2014
Mar. 31, 2014
Schedule Of Intangible Assets [Line Items]      
Indefinite Lived Intangible Assets, Gross Carrying Amount $ 12,800us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill $ 12,800us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill  
Indefinite Lived Intangible Assets, Accumulated Amortization 0ghm_IndefiniteLivedIntangiblesAccumulatedAmortization 0ghm_IndefiniteLivedIntangiblesAccumulatedAmortization  
Indefinite Lived Intangible Assets, Net Carrying Amount 12,800us-gaap_IntangibleAssetsNetExcludingGoodwill 12,800us-gaap_IntangibleAssetsNetExcludingGoodwill  
Permits [Member]      
Schedule Of Intangible Assets [Line Items]      
Indefinite Lived Intangible Assets, Gross Carrying Amount 10,300us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
10,300us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
 
Indefinite Lived Intangible Assets, Accumulated Amortization 0ghm_IndefiniteLivedIntangiblesAccumulatedAmortization
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
0ghm_IndefiniteLivedIntangiblesAccumulatedAmortization
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
 
Indefinite Lived Intangible Assets, Net Carrying Amount 10,300us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
10,300us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= ghm_NuclearPermitsMember
 
Tradename [Member]      
Schedule Of Intangible Assets [Line Items]      
Indefinite Lived Intangible Assets, Gross Carrying Amount 2,500us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
2,500us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
 
Indefinite Lived Intangible Assets, Accumulated Amortization 0ghm_IndefiniteLivedIntangiblesAccumulatedAmortization
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
0ghm_IndefiniteLivedIntangiblesAccumulatedAmortization
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
 
Indefinite Lived Intangible Assets, Net Carrying Amount 2,500us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
2,500us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
 
Customer Relationships [Member]      
Schedule Of Intangible Assets [Line Items]      
Finite-Lived Intangible Assets, Gross Carrying Amount 2,700us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
  2,700us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Finite-Lived Intangible Assets, Accumulated Amortization 772us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
  592us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Finite-Lived Intangible Assets, Net Carrying Amount $ 1,928us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
  $ 2,108us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember