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Employee Benefit Plans - Summary of Weighted Average Asset Allocation of Plan Assets by Asset Category (Detail) | 12 Months Ended |
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Mar. 31, 2015 | Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items] |  |
Target Plan Asset Allocations | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Plan Asset Allocations Range Minimum | 50.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |
Target Plan Asset Allocations Range Maximum | 70.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |
Target Plan Asset Allocations | 66.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember | 66.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember
Debt Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Plan Asset Allocations Range Minimum | 20.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember |
Target Plan Asset Allocations Range Maximum | 50.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember |
Target Plan Asset Allocations | 34.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember | 34.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_DebtSecuritiesMember
Other, including cash [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Plan Asset Allocations Range Minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = ghm_OtherSecuritiesMember |
Target Plan Asset Allocations Range Maximum | 10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = ghm_OtherSecuritiesMember |

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- Definition Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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