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Organization - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016 Employee Country Customer	Oct. 25, 2012 shares
Organization [Line Items]
Number of professionals around the globe, minimum | Employee	70,000
Number of countries in which entity operates | Country	25
Common stock shares purchased by affiliates of Bain Capital Partners | shares		67,750,678
Minimum | Fortune Global 500
Organization [Line Items]
Number of clients | Customer	100

X

- Definition

Number of persons employed by the Entity

+ References

No definition available.

+ Details

Name:	dei_EntityNumberOfEmployees
Namespace Prefix:	dei_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Definition

It represents the number of common shares of the company purchased by certain parties, in accordance with the share purchase agreement entered into with the selling shareholders.

+ References

No definition available.

+ Details

Name:	g_CommonShareSaleTransaction
Namespace Prefix:	g_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

It represents the total number of clients to whom services are delivered.

+ References

No definition available.

+ Details

Name:	g_NumberOfClients
Namespace Prefix:	g_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This line item represents in the information pertaining to organization of the company.

+ References

No definition available.

+ Details

Name:	g_OrganizationLineItems
Namespace Prefix:	g_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of countries in which the entity operates as of balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfCountriesInWhichEntityOperates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MajorCustomersAxis=g_FortuneGlobalFiveHundredMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: