Changes in Accounting Estimate (Details) (USD $)		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013 Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member] Vessel	Jun. 30, 2013 Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member]	Mar. 31, 2013 Extended Economic Life of Both Mobile Alabama and Coatzacoalcos [Member]	Jun. 30, 2013 Extended Economic Life of Both Mobile Alabama and Coatzacoalcos [Member]	Mar. 31, 2012 Change in Salvage Value [Member] Vessel	Jun. 30, 2012 Change in Salvage Value [Member]
Changes in Accounting Estimate [Abstract]
Period between salvage value reviews	3 years
Period used to determine reasonableness of salvage values	3 years
Change in Accounting Estimate [Line Items]
Number of vessels for which salvage values were adjusted						8
Number of vessels to have extended life		2
Decrease in future depreciation expense		$ 1.1				$ 3,800,000
Increase in pre-tax income			540,466		537,661		1,890,000
Increase in net income			$ 540,466		$ 537,661		$ 1,890,000
Increase in basic earnings per share (in dollars per share)			$ 0.07		$ 0.07		$ 0.26
Increase in diluted earnings per share (in dollars per share)			$ 0.07		$ 0.07		$ 0.26
Extended amortization period of assets				5 years