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Fair Value Measurements - Roll-Forward of Fair Value of Deferred Compensation Plan Assets Categorized as Level 3 in Fair Value Hierarchy (Parenthetical) (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Fair Value Disclosures [Abstract]
Accrued liability	$ 7,248

X

- Definition

Deferred compensation, accrued liability.

+ References

No definition available.

+ Details

Name:	g_DeferredCompensationAccruedLiability
Namespace Prefix:	g_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

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