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Summary of Restricted Share Units Granted (Parenthetical) (Detail) - shares | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Jan. 31, 2016
Restricted Share Units (RSUs) |  |
Schedule Of Activity Related To Restricted Shares And Restricted Share [Line Items] |  |
RSUs settled on vesting by issuing shares (net of minimum tax withholding) | 3,228 | 91,963

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- Definition Schedule Of Activity Related To Restricted Shares And Restricted Share [Line Items] + References No definition available. + Details
Name: | g_ScheduleOfActivityRelatedToRestrictedSharesAndRestrictedShareLineItems
Namespace Prefix: | g_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The number of restricted shares issued as compensation, net of shares for the payment of withholding taxes. This element is to be used only if shares are used in lieu of cash to satisfy all or a portion of withholding taxes. + References No definition available. + Details
Name: | us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |