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Product Warranty Liability - Reconciliation of the Changes in Product Warranty Liability (Detail) - USD ($) $ in Thousands | 3 Months Ended
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Jun. 30, 2020 | Jun. 30, 2019
Guarantees [Abstract] |  |
Balance at beginning of period | $ 359 | $ 366
(Income) expense for product warranties | (19) | 27
Product warranty claims paid | (35) | (35)
Balance at end of period | $ 305 | $ 358

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- Definition The (income) expense (included) charged (in) against earnings for the period pertaining to standard and extended warranties on the entity's goods and services granted to customers. + References No definition available. + Details
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Namespace Prefix: | ghm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_GuaranteesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Carrying value as of the balance sheet date of obligations incurred through that date and payable for estimated claims under standard and extended warranty protection rights granted to customers. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15(5)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c)(5) -URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

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- Definition Amount of decrease in the standard product warranty accrual from payments made in cash or in kind to satisfy claims under the terms of the standard product warranty. Excludes extended product warranties. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph ((c)(2)) -URI http://asc.fasb.org/extlink&oid=68068213&loc=d3e12565-110249 + Details
Name: | us-gaap_StandardProductWarrantyAccrualPayments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration