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Long-Term Debt (Narrative) (Details) - USD ($)		3 Months Ended	6 Months Ended	9 Months Ended
Long-Term Debt (Narrative) (Details) - USD ($)	Jun. 24, 2016	Mar. 31, 2015	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Nov. 04, 2014	Jun. 04, 2012
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 199,000,000								$ 210,000,000
Net book value of related assets				$ 132,178,000
Fixed Charge Coverage Ratio				1.25%
Revolving credit facility				$ 60,000,000
Letters of credit outstanding				68,893,000
Letter of credit remaining borrowing capacity				70,107,000
Borrowed aggregate principal amount						$ 2,640,000	$ 47,360,000
Purchases of property and equipment				58,842,000	$ 53,394,000
Long-term Debt, Current Maturities				11,745,000		$ 7,506,000
7.375% Senior Notes Due In 2019 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount				$ 275,000,000
Debt Instrument, Interest Rate, Stated Percentage				7.375%
Maturity date				Feb. 01, 2019
If Redeemed In 2015 [Member] \| 7.375% Senior Notes Due In 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage				103.70%
If Redeemed In 2016 [Member] \| 7.375% Senior Notes Due In 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage				101.80%
If Redeemed After 2016 [Member] \| 7.375% Senior Notes Due In 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage				100.00%
Term Loan Facility [Member]
Debt Instrument [Line Items]
Net book value of related assets				$ 47,975,000
Debt instrument, face amount								$ 50,000,000
Debt Instrument, Term				5 years
Term Loan Facility Bear Interest Fixed Rate				4.655%
Debt Instrument, Interest Rate, Increase (Decrease)				2.00%
Percentage of principal to be repaid yearly during first two years of term				10.00%
Percentage of principal to be repaid yearly during third and fourth years of term				20.00%
Percentage of principal to be repaid during fifth year of term				25.00%
Vessels and Ancillary Equipment [Member]
Debt Instrument [Line Items]
Purchases of property and equipment		$ 15,569,000
Operating leases expiration year				2023
Long-term Debt, Current Maturities		2,442,000
Long-term Debt, Excluding Current Maturities		$ 15,275,000
Vessels and Ancillary Equipment [Member] \| 5.75% Senior Notes Due In 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.75%
Maximum [Member]
Debt Instrument [Line Items]
Total leverage ratio for fiscal quarter ending June 30, 2016				5.00%
Total leverage ratio for fiscal quarter ending September 30, 2016				4.75%
Total leverage ratio for any fiscal quarter ending thereafter				4.50%
Aggregate capital expenditures for fiscal year ending December 30, 2016				$ 105,000,000
Aggregate capital expenditures during any fiscal year thereafter				$ 45,000,000
Maximum [Member] \| LIBOR Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			3.50%
Maximum [Member] \| Base Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			2.50%
Minimum [Member] \| LIBOR Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.50%
Minimum [Member] \| Base Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			0.50%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity									199,000,000
Line of credit facility optional increase capacity									15,000,000
Revolving credit facility, interest rate description				The Credit Agreement is collateralized by a substantial portion of the Company’s operating equipment with a net book value at September 30, 2016 of $132,178. Depending on the Company’s consolidated total leverage ratio (as defined in the Credit Agreement and further described below), borrowings under the revolving credit facility will bear interest at the option of the Company at either the LIBOR rate plus a margin between 1.50% and 3.50% or a base rate plus a margin between 0.50% and 2.50%.
Revolving credit facility, maturity date				Jun. 02, 2017
Repayments of Lines of Credit	$ 10,000,000
Multicurrency [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity									50,000,000
Swingline Loans [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity									$ 10,000,000