XML 48 R37.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Investments - Additional Information (Detail) | 3 Months Ended
--- | ---
Jun. 30, 2018
Minimum [Member] |
Schedule of Held-to-maturity Securities [Line Items] |
Treasury with original maturities period | 3 months
Maximum [Member] |
Schedule of Held-to-maturity Securities [Line Items] |
Investment maturity date range end | Dec. 27, 2018
Treasury with original maturities period | 1 year

X
- Definition Original maturity period for held to maturity securities. + References No definition available. + Details
Name: | ghm_HeldToMaturitySecuritiesOriginalMaturityPeriod
Namespace Prefix: | ghm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maturity date of the investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
Name: | invest_InvestmentMaturityDate1
Namespace Prefix: | invest_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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