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Stock-Based Compensation (Details 2) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
Mar. 30, 2015
RSU [Roll Forward]    
Outstanding and unvested, beginning of period 263,084us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber  
Granted 82,120us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod  
Vested (2,050)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod  
Forfeited (325)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod  
Outstanding and unvested, end of period 342,829us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber  
Outstanding and unvested, beginning of period, Weighted average grant date fair value $ 25.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue  
Granted, Weighted average grant date fair value $ 36.71us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue  
Vested, Weighted average grant date fair value $ 15.36us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue  
Forfeited, Weighted average grant date fair value $ 25.92us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue  
Outstanding and unvested, end of period, Weighted average grant date fair value $ 27.86us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue  
Restricted Stock Units (RSUs) [Member]    
RSU [Roll Forward]    
Vesting period 5 years  
Restricted Stock Units (RSUs) [Member] | Performance-based Restricted Stock Units [Member]    
RSU [Roll Forward]    
Granted   52,476us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_VestingAxis
= gpx_PerformancebasedRestrictedStockUnitsMember
Restricted Stock Units (RSUs) [Member] | Time-based Restricted Stock Units [Member]    
RSU [Roll Forward]    
Granted   29,644us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_VestingAxis
= gpx_TimebasedRestrictedStockUnitsMember