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Amtrak Agreement - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Apr. 04, 2012
Dispute Settlement Agreement [Line Items]      
Dispute settlement gross amount $ 1,219pwx_DisputeSettlementGrossAmount $ 1,994pwx_DisputeSettlementGrossAmount  
Amtrak Agreement [Member]      
Dispute Settlement Agreement [Line Items]      
Dispute settlement gross amount     5,577pwx_DisputeSettlementGrossAmount
/ us-gaap_EnergyAxis
= pwx_AmtrakAgreementMember
Amtrak Agreement [Member] | License Arrangement [Member]      
Dispute Settlement Agreement [Line Items]      
Dispute settlement gross amount     179pwx_DisputeSettlementGrossAmount
/ us-gaap_EnergyAxis
= pwx_AmtrakAgreementMember
/ us-gaap_GainContingenciesByNatureAxis
= us-gaap_NonsoftwareLicenseArrangementMember
Amtrak Agreement [Member] | Track Material [Member]      
Dispute Settlement Agreement [Line Items]      
Dispute settlement gross amount     684pwx_DisputeSettlementGrossAmount
/ us-gaap_EnergyAxis
= pwx_AmtrakAgreementMember
/ us-gaap_GainContingenciesByNatureAxis
= pwx_TrackMaterialMember
Amtrak Agreement [Member] | Inter Entity Outstanding Adjustment [Member]      
Dispute Settlement Agreement [Line Items]      
Dispute settlement gross amount     2,143pwx_DisputeSettlementGrossAmount
/ us-gaap_EnergyAxis
= pwx_AmtrakAgreementMember
/ us-gaap_GainContingenciesByNatureAxis
= pwx_InterEntityOutstandingAdjustmentMember
Amtrak Agreement [Member] | Mileage Charges Freight [Member]      
Dispute Settlement Agreement [Line Items]      
Dispute settlement gross amount     $ 2,571pwx_DisputeSettlementGrossAmount
/ us-gaap_EnergyAxis
= pwx_AmtrakAgreementMember
/ us-gaap_GainContingenciesByNatureAxis
= pwx_MileageChargesFreightMember