Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Details 1)

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Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Details 1)
 12 Months Ended
Dec. 31, 2017
Ships  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life 30 years
Computer hardware and software | Minimum  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life 3 years
Computer hardware and software | Maximum  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life 10 years
Other property and equipment | Minimum  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life 3 years
Other property and equipment | Maximum  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life 40 years
Leasehold improvements  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life Shorter of lease term or asset life
Ship improvements  
Property, Plant and Equipment [Line Items]  
Property and equipment, useful life Shorter of asset life or life of the ship