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Reserve for Doubtful Receivables (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
Provisions for Doubtful Accounts [Line Items] |  |  |
Opening Balance | $ 11,530 | $ 15,192 | $ 15,192
Additions charged to cost and expense | 3,120 | $ 872 | 2,449
Deductions/effect of exchange rate fluctuations | 27 |  | (6,111)
Closing Balance | $ 14,677 |  | $ 11,530

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- Definition Allowance for Doubtful Accounts Receivable Deductions and Foreign Currency Translation Adjustments + References No definition available. + Details
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Namespace Prefix: | g_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Provisions for Doubtful Accounts [Line Items] + References No definition available. + Details
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Namespace Prefix: | g_
Data Type: | xbrli:stringItemType
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- Definition For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

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- Definition Amount of expense related to write-down of receivables to the amount expected to be collected. Includes, but is not limited to, accounts receivable and notes receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.5) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 + Details
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Namespace Prefix: | us-gaap_
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