Organization - Additional Information (Detail)	Jun. 30, 2017 Employee Country	Oct. 25, 2012 shares
Accounting Policies [Abstract]
Number of employees around the globe, minimum \| Employee	77,000
Number of countries in which entity operates \| Country	20
Common stock shares purchased by affiliates of Bain Capital Partners \| shares		67,750,678

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.

No definition available.

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