Long-Term Debt (Details Textual) - USD ($) $ in Thousands				3 Months Ended
	Mar. 27, 2018	Jan. 08, 2018	May 04, 2016	Mar. 31, 2018	Mar. 31, 2017
Long-Term Debt (Textual)
Deferred financing costs charged to interest expense				$ 608	$ 552
Credit agreement, description				The Company capitalized $4.2 million related to lender and third-party fees in connection with the Third Amended and Restated Credit Agreement. In addition, the entry into the Third Amended and Restated Credit Agreement was considered a debt modification with a partial extinguishment, as a result the Company incurred costs of $1.0 million.
Credit Facility [Member]
Long-Term Debt (Textual)
Maximum borrowing capacity	$ 200,000
Increase in line of credit facility	$ 25,000
Credit facility, expiration date	Mar. 27, 2025
Deferred financing costs charged to interest expense				$ 600	$ 600
Credit agreement, description	Borrowings under the Term Facility will bear interest at an adjusted ICE Benchmark administration LIBOR plus a spread of 3.50%, which steps down to 3.25% if the Company's debt rating from Moody's and S&P are both B1 (stable) or better and BB (negative) or better, respectively. The interest rate at March 31, 2018 is 5.95%.
Revolving Facility [Member]
Long-Term Debt (Textual)
Increase in line of credit facility	$ 5,000
Description of interest rate	Borrowings under the Revolving Facility will bear interest at an adjusted ICE Benchmark administration LIBOR plus a spread of 3.00%, or, at the option of the Company, an alternative base rate plus a spread of 2.00%. The Company is also required to pay a 0.5% annual commitment fee on undrawn amounts under the Revolving Credit Facility.
Credit facility, expiration date	Mar. 27, 2023
Revolving credit facility	$ 45,000
Note payable [Member]
Long-Term Debt (Textual)
Outstanding principal amount			$ 2,500
Debt maturity date			Dec. 31, 2020
Promissory note interest rate			1.44%
Restated Credit Facility [Member]
Long-Term Debt (Textual)
Credit agreement, description	The Restated Credit Agreement (i) requires the Company to satisfy certain financial covenants as set forth in the Amended Credit Agreement; (ii) limits the amount of indebtedness the Company may incur; (iii) limits the amount the Company may spend in connection with certain types of investments; (iv) requires the delivery of certain periodic financial statements and an operating budget and (v) requires the mortgaged vessels and related inventory to be maintained in good working condition.
Senior Secured Credit Agreement [Member]
Long-Term Debt (Textual)
Outstanding principal amount		$ 107,700
Description of interest rate		The Borrower's election, the loan will bear interest either at a fixed interest rate effectively equal to 5.78% or a floating interest rate equal to three-month LIBOR plus a margin of 3.00% per annum. The loan will amortize quarterly based on a twelve-year profile, with 70% maturing over twelve years from drawdown, and 30% maturing over five years from drawdown.
Credit agreement, description		The purpose of providing financing for up to 80% of the purchase price of the Company's new ice class vessel, the National Geographic Endurance, targeted to be completed in January 2020. Seventy percent of the loan will be guaranteed by Garantiinstituttet for Eksportkreditt, the official export credit agency of Norway. If drawn upon, the loan will be made at the time of delivery of the vessel.