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Fair Value of Financial Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value of Financial Instruments [Line Items]    
Period end valuation 0gnw_PeriodEndValuation  
GMWB non-performance risk impact $ 74gnw_GmwbNonperformanceRiskImpact $ 46gnw_GmwbNonperformanceRiskImpact
Minimum    
Fair Value of Financial Instruments [Line Items]    
Percentage of changes in fair value of fixed maturity, equity and trading securities each month 10.00%gnw_PercentageOfChangesInFairValueFixedMaturityEquityTradingSecurities
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