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Leases (Narrative) (Details) (USD $)
0 Months Ended 12 Months Ended 0 Months Ended
Nov. 30, 2012
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Nov. 27, 2012
Dec. 27, 2012
Feb. 22, 2012
Jun. 30, 2013
Leases [Line Items]                
Vessels, gross   $ 520,026,000ish_VesselsGross $ 582,416,000ish_VesselsGross          
Repayment of debt from proceeds of sale and leaseback transaction       (54,500,000)us-gaap_ProceedsFromRepaymentsOfDebt        
Vessel repurchased, amount   55,600,000ish_VesselRepurchasedAmount            
Unamortized deferred gain recorded as reduction in cost basis of purchased asset   11,200,000ish_UnamortizedDeferredGain            
Sale-leaseback net proceeds 63,000,000us-gaap_SaleLeasebackTransactionNetProceeds              
Office space lease agreement term   20 years            
Leasehold improvements   26,348,000us-gaap_LeaseholdImprovementsGross 26,348,000us-gaap_LeaseholdImprovementsGross          
Potential lease termination expense   3,100,000us-gaap_GainLossOnContractTermination            
Rent expense   21,100,000us-gaap_LeaseAndRentalExpense 20,000,000us-gaap_LeaseAndRentalExpense 11,200,000us-gaap_LeaseAndRentalExpense        
BMO Harris Equipment Finance Company [Member]                
Leases [Line Items]                
Sale leaseback, gross proceeds         32,000,000us-gaap_SaleLeasebackTransactionGrossProceeds
/ dei_LegalEntityAxis
= ish_LessorOneMember
     
Lease term         7 years      
Gain on sale-leaseback         8,000,000us-gaap_SaleLeasebackTransactionDeferredGainGross
/ dei_LegalEntityAxis
= ish_LessorOneMember
     
Capital Source Bank [Member]                
Leases [Line Items]                
Sale leaseback, gross proceeds         31,000,000us-gaap_SaleLeasebackTransactionGrossProceeds
/ dei_LegalEntityAxis
= ish_LessorTwoMember
     
Lease term         6 years      
Gain on sale-leaseback         11,700,000us-gaap_SaleLeasebackTransactionDeferredGainGross
/ dei_LegalEntityAxis
= ish_LessorTwoMember
     
BB&T Equipment Finance [Member]                
Leases [Line Items]                
Sale leaseback, gross proceeds           32,000,000us-gaap_SaleLeasebackTransactionGrossProceeds
/ dei_LegalEntityAxis
= ish_LessorThreeMember
   
Lease term           6 years    
Wells Fargo Bank Northwest [Member]                
Leases [Line Items]                
Sale leaseback, gross proceeds             59,000,000us-gaap_SaleLeasebackTransactionGrossProceeds
/ dei_LegalEntityAxis
= ish_LessorFourMember
 
Lease term             10 years  
Gain on sale-leaseback             14,900,000us-gaap_SaleLeasebackTransactionDeferredGainGross
/ dei_LegalEntityAxis
= ish_LessorFourMember
 
New York Office [Member]                
Leases [Line Items]                
Office space lease agreement term   10 years            
Period of lease agreement that is free of rent   9 months            
Leasehold improvements   503,000us-gaap_LeaseholdImprovementsGross
/ us-gaap_StatementGeographicalAxis
= stpr_NY
           
Shanghai Office [Member]                
Leases [Line Items]                
Office space lease agreement term   2 years            
Tampa [member]                
Leases [Line Items]                
Office space lease agreement term   5 years            
Property Subject to Operating Lease [Member]                
Leases [Line Items]                
Vessels, gross               16,200,000ish_VesselsGross
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= us-gaap_PropertySubjectToOperatingLeaseMember
Mobile Corporate Office Lease [Member]                
Leases [Line Items]                
Leasehold improvements   730,000us-gaap_LeaseholdImprovementsGross
/ ish_LeaseArrangementTypeAxis
= ish_MobileCorporateOfficeLeaseMember
           
Mobile Corporate Office Lease [Member] | Leasehold Improvements [Member]                
Leases [Line Items]                
Incentive from Lessor   $ 1,400,000us-gaap_IncentiveFromLessor
/ ish_LeaseArrangementTypeAxis
= ish_MobileCorporateOfficeLeaseMember
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LeaseholdImprovementsMember
           
Mobile Corporate Office Lease [Member] | Minimum [Member]                
Leases [Line Items]                
Early termination operating lease term   10 years            
Mobile Corporate Office Lease [Member] | Maximum [Member]                
Leases [Line Items]                
Early termination operating lease term   15 years