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Stock-Based Compensation Performance Based Restricted Stock Units (Details) (Performance Based Restricted Stock Units, USD $)
12 Months Ended
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Performance based restricted stock units, performance period 3 years
Performance based restricted stock units, nonvested, number of shares [Roll Forward]  
Non-vested at beginning of year 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Granted 14,424us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Vested 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Forfeited 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
Non-vested at end of year 14,424us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Performance based restricted stock units, nonvested, weighted average grant date fair value [Abstract]  
Non-vested at beginning of year $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Granted $ 42.39us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Vested $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
Forfeited $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
Non-vested at end of year $ 42.39us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Statement [Line Items]  
Monte Carlo valuation model Monte Carlo valuation model
Expected dividend yield 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Average correlation coefficient of peer group and S&P companies 0.6gwr_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueAssumptionsCorrelationCoefficient
Risk-free interest rate 0.81%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Expected term (in years) 3 years
Volatility of the Company's common stock
 
Statement [Line Items]  
Expected volatility 25.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_VestingAxis
= us-gaap_ShareBasedCompensationAwardTrancheOneMember
Average volatility of peer group and S&P companies
 
Statement [Line Items]  
Expected volatility 29.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_VestingAxis
= us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Minimum [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Performance based restricted stock units, vesting rights 0.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Performance based restricted stock units, vesting rights 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember