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Other Operating (Income) Expense, Net - Schedule of Other Operating (Income) Expense, Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Other Income And Expenses [Abstract]
Other operating (income) expense	$ (320)	$ 86
Other operating (income) expense, net	$ (320)	$ 86

X

- Definition

Amount of other operating income (expenses), not previously categorized, from items that are associated with the entity's operation.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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

X

- Definition

The net amount of other operating income and expenses, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operations.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherOperatingIncomeExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration