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The Company and Its Accounting Policies - Reconciliation of Numerators and Denominators of Basic and Diluted Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2015 | Mar. 31, 2014 | Mar. 31, 2013
Numerator: |  |  |  |  |  |  |  |  |  |  |
Net income | $ 4,165 us-gaap_NetIncomeLoss | $ 3,992 us-gaap_NetIncomeLoss | $ 4,186 us-gaap_NetIncomeLoss | $ 2,392 us-gaap_NetIncomeLoss | $ 2,317 us-gaap_NetIncomeLoss | $ 1,431 us-gaap_NetIncomeLoss | $ 2,589 us-gaap_NetIncomeLoss | $ 3,808 us-gaap_NetIncomeLoss | $ 14,735 us-gaap_NetIncomeLoss | $ 10,145 us-gaap_NetIncomeLoss | $ 11,148 us-gaap_NetIncomeLoss
Denominator: |  |  |  |  |  |  |  |  |  |  |
Weighted common shares outstanding |  |  |  |  |  |  |  |  | 10,123 ghm_WeightedAverageCommonSharesOutstanding | 10,056 ghm_WeightedAverageCommonSharesOutstanding | 9,984 ghm_WeightedAverageCommonSharesOutstanding
Share equivalent units ("SEUs") outstanding |  |  |  |  |  |  |  |  |  | 14 us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable | 43 us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable
Weighted average common shares and SEUs outstanding |  |  |  |  |  |  |  |  | 10,123 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 10,070 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 10,027 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Basic income per share | $ 0.41 us-gaap_EarningsPerShareBasic | $ 0.39 us-gaap_EarningsPerShareBasic | $ 0.41 us-gaap_EarningsPerShareBasic | $ 0.24 us-gaap_EarningsPerShareBasic | $ 0.23 us-gaap_EarningsPerShareBasic | $ 0.14 us-gaap_EarningsPerShareBasic | $ 0.26 us-gaap_EarningsPerShareBasic | $ 0.38 us-gaap_EarningsPerShareBasic | $ 1.46 us-gaap_EarningsPerShareBasic | $ 1.01 us-gaap_EarningsPerShareBasic | $ 1.11 us-gaap_EarningsPerShareBasic
Numerator: |  |  |  |  |  |  |  |  |  |  |
Net income | $ 4,165 us-gaap_NetIncomeLoss | $ 3,992 us-gaap_NetIncomeLoss | $ 4,186 us-gaap_NetIncomeLoss | $ 2,392 us-gaap_NetIncomeLoss | $ 2,317 us-gaap_NetIncomeLoss | $ 1,431 us-gaap_NetIncomeLoss | $ 2,589 us-gaap_NetIncomeLoss | $ 3,808 us-gaap_NetIncomeLoss | $ 14,735 us-gaap_NetIncomeLoss | $ 10,145 us-gaap_NetIncomeLoss | $ 11,148 us-gaap_NetIncomeLoss
Denominator: |  |  |  |  |  |  |  |  |  |  |
Weighted average common shares and SEUs outstanding |  |  |  |  |  |  |  |  | 10,123 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 10,070 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 10,027 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Stock options outstanding |  |  |  |  |  |  |  |  | 20 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements | 34 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements | 24 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Weighted average common and potential common shares outstanding |  |  |  |  |  |  |  |  | 10,143 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 10,104 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 10,051 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Diluted income per share | $ 0.41 us-gaap_EarningsPerShareDiluted | $ 0.39 us-gaap_EarningsPerShareDiluted | $ 0.41 us-gaap_EarningsPerShareDiluted | $ 0.24 us-gaap_EarningsPerShareDiluted | $ 0.23 us-gaap_EarningsPerShareDiluted | $ 0.14 us-gaap_EarningsPerShareDiluted | $ 0.26 us-gaap_EarningsPerShareDiluted | $ 0.38 us-gaap_EarningsPerShareDiluted | $ 1.45 us-gaap_EarningsPerShareDiluted | $ 1.00 us-gaap_EarningsPerShareDiluted | $ 1.11 us-gaap_EarningsPerShareDiluted

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