Changes in Accounting Estimate (Details) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended
	Mar. 31, 2012 item	Sep. 30, 2012	Sep. 30, 2013	Mar. 31, 2013 Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member] item	Sep. 30, 2013 Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member]	Sep. 30, 2013 Extended Economic Life of Both Mobile Alabama and Coatzacoalcos [Member]
Change in Accounting Estimate [Line Items]
Period between salvage value reviews			3 years
Period used to determine reasonableness of salvage values			3 years
Number of vessels for which salvage values were adjusted	8
Decrease in future depreciation expense		$ 3,800,000			$ 1,100,000
Increase in pre-tax income		2,835,000			811,000	811,000
Increase in net income		$ 2,835,000			$ 811,000	$ 811,000
Increase in basic earnings per share (in dollars per share)		$ 0.39			$ 0.11	$ 0.11
Number of vessels to have extended life				2
Extended amortization period of assets						5 years