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v3.20.1

Subsequent Events - Additional Information (Detail) - Letter of Credit [Member] - USD ($) $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
May 01, 2020 | Mar. 31, 2019 | Oct. 08, 2019
HSBC Bank USA [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Line of credit |  | $ 5,000 | $ 10,000
HSBC Bank USA [Member] | Subsequent Event [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Line of credit | $ 14,000 |  |
Revolving Credit Facility [Member] | Subsequent Event [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Annual facility fee | $ 5 |  |
Debt instrument, basis spread on variable rate | 3.00% |  |
Revolving Credit Facility [Member] | Minimum [Member] | Subsequent Event [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Fee for outstanding letters of credit | 0.75% |  |
Revolving Credit Facility [Member] | Maximum [Member] | Subsequent Event [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Fee for outstanding letters of credit | 0.85% |  |
Revolving Credit Facility [Member] | HSBC Bank USA [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Annual facility fee |  | $ 5 |
Debt instrument, basis spread on variable rate |  | 3.00% |
Revolving Credit Facility [Member] | HSBC Bank USA [Member] | Minimum [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Fee for outstanding letters of credit |  | 0.65% |
Revolving Credit Facility [Member] | HSBC Bank USA [Member] | Maximum [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Fee for outstanding letters of credit |  | 0.75% |

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- Definition The fee, expressed as a percentage of the standby letter of credit amount, incurred for the issuance of a standby letter of credit. + References No definition available. + Details
Name: | ghm_LetterOfCreditCommitmentFeePercentage
Namespace Prefix: | ghm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of the flat fee for the line of credit facility. + References No definition available. + Details
Name: | ghm_LineOfCreditFee
Namespace Prefix: | ghm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LineOfCreditFacilityAxis=ghm_HSBCBankUSAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ShortTermDebtTypeAxis=us-gaap_LetterOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |