Quarterly report pursuant to Section 13 or 15(d)

Income Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Income Per Share (Detail)

v2.4.1.9
Income Per Share - 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 9 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Numerator:        
Net income $ 3,992us-gaap_NetIncomeLoss $ 1,431us-gaap_NetIncomeLoss $ 10,570us-gaap_NetIncomeLoss $ 7,828us-gaap_NetIncomeLoss
Denominator:        
Weighted common shares outstanding 10,127ghm_WeightedAverageCommonSharesOutstanding 10,070ghm_WeightedAverageCommonSharesOutstanding 10,119ghm_WeightedAverageCommonSharesOutstanding 10,045ghm_WeightedAverageCommonSharesOutstanding
Share equivalent units ("SEUs") outstanding       18ghm_WeightedAverageNumberOfShareEquivalentUnitsOutstandingBasic
Weighted average common shares and SEUs outstanding 10,127us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 10,070us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 10,119us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 10,063us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Basic income per share $ 0.39us-gaap_EarningsPerShareBasic $ 0.14us-gaap_EarningsPerShareBasic $ 1.04us-gaap_EarningsPerShareBasic $ 0.78us-gaap_EarningsPerShareBasic
Numerator:        
Net income $ 3,992us-gaap_NetIncomeLoss $ 1,431us-gaap_NetIncomeLoss $ 10,570us-gaap_NetIncomeLoss $ 7,828us-gaap_NetIncomeLoss
Denominator:        
Weighted average shares and SEUs outstanding 10,127us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 10,070us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 10,119us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 10,063us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Stock options outstanding 22us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 37us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 23us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 36us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Weighted average common and potential common shares outstanding 10,149us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 10,107us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 10,142us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 10,099us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Diluted income per share $ 0.39us-gaap_EarningsPerShareDiluted $ 0.14us-gaap_EarningsPerShareDiluted $ 1.04us-gaap_EarningsPerShareDiluted $ 0.78us-gaap_EarningsPerShareDiluted