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Stock-Based Compensation - Summary of Performance Units Activity (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2020 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net settlement on vesting of performance units, shares	902,532
Performance Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vested, number of shares (Units)	1,496,377	[1]
[1] 1,496,377 PSUs that vested during the period were net settled upon vesting by issuing  902,532  shares (net of minimum statutory tax withholding).

X

- Definition

Shares awarded to employees, net of shares for the payment of withholding taxes for meeting certain performance targets. 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:	g_PerformanceUnitSharesIssuedNetOfTaxWithholdings
Namespace Prefix:	g_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: