Commitments and Contingencies (Details Textual) - USD ($) $ in Millions		1 Months Ended	3 Months Ended
	Dec. 02, 2015	Nov. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Nichols Brothers Boat Builders [Member]
Commitments and Contingencies (Textual)
Vessel construction agreements, description	On December 2, 2015, the Company entered into two separate Vessel Construction Agreements, (collectively, the "Agreements") with Ice Floe, LLC, a Washington limited liability company doing business as Nichols Brothers Boat Builders (the "Builder"). The Agreements provide for the Builder to construct two new 236-foot 100-passenger cruise vessels.
National Geographic [Member]
Commitments and Contingencies (Textual)
Risk of loss or damage, description			The Company amended the agreement for the second vessel, the National Geographic Venture, in October 2017. The current contract price is $57.3 million and the vessel is scheduled to be completed in the fourth quarter of 2018, subject to extension for certain events, such as change orders. As of March 31, 2018, the Company has paid Ice Floe, LLC $34.8 million related to the National Geographic Venture. The Company may terminate the applicable Agreement in the event the builder fails to deliver the vessel within 180 days of the applicable due date or the builder becomes insolvent or otherwise bankrupt. The Agreement also contains customary representations, warranties, covenants and indemnities.
National Geographic [Member] | Royalty Agreement [Member]
Commitments and Contingencies (Textual)
Royalty expense			$ 1.6	$ 1.2
Balance outstanding			1.6		$ 1.7
World Wildlife Fund [Member] | Royalty Agreement [Member]
Commitments and Contingencies (Textual)
Royalty expense			$ 0.2	$ 0.2
Ulstein Verft [Member]
Commitments and Contingencies (Textual)
Vessel construction agreements, description		The Company entered into an agreement with Ulstein Verft to construct a polar ice class vessel, the National Geographic Endurance, with a total purchase price of 1,066.0 million Norwegian Kroner (NOK). Subsequently, the Company exercised its right to make payments in United States Dollars, which resulted in a purchase price of $134.6 million, including hedging costs. The purchase price is subject to potential adjustments from contract specifications for variations in speed, deadweight, fuel consumption and delivery date, and is due in installments. The first twenty percent of the purchase price was paid shortly after execution of the Agreement with the remaining eighty percent due upon delivery and acceptance of the vessel. The vessel is targeted to be delivered in January 2020, with potential accelerated delivery to November 2019. The contract also includes options to build two additional ice class vessels.
Cruise vessels at a purchase price		$ 134.6