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Income Taxes - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2014
Dec. 31, 2013
Income Tax Benefit [Line Items]        
Income Tax rate 14.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 54.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 14.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 28.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Railroad Track Maintenance Credits in the cumulative amount     $ 4,491us-gaap_TaxCreditCarryforwardAmount  
Maximum period to offset future income tax liabilities     20 years  
State and Local Jurisdiction [Member] | Massachusetts [Member]        
Income Tax Benefit [Line Items]        
Income Tax rate     8.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ us-gaap_StatementGeographicalAxis
= stpr_MA		 6.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ us-gaap_StatementGeographicalAxis
= stpr_MA