Summary of Significant Accounting Policies (Details 3)	3 Months Ended
	Mar. 31, 2018
Vessels and vessel improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	25 years
Vessels and vessel improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	15 years
Furniture and equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Computer hardware and software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold improvements, including port facilities [Member]
Property, Plant and Equipment [Line Items]
Leasehold improvements, including port facilities	Shorter of lease term or related asset life