Changes in Accounting Estimate (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 Vessel	Mar. 31, 2012
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]
Decrease in future depreciation expense	937,000
Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member]
Change in Accounting Estimate [Line Items]
Number of vessels to have extended life	2
Increase in pre-tax income	234,265
Increase in net income	234,265
Increase in basic earnings per share (in dollars per share)	0.03
Increase in diluted earnings per share (in dollars per share)	0.03
Extended Economic Life of Both Mobile Alabama and Coatzacoalcos [Member]
Change in Accounting Estimate [Line Items]
Increase in pre-tax income	267,345
Increase in net income	267,345
Increase in basic earnings per share (in dollars per share)	0.04
Increase in diluted earnings per share (in dollars per share)	0.04
Extended amortization period of assets	5 years
Change in Salvage Value [Member]
Change in Accounting Estimate [Line Items]
Number of vessels for which salvage values were adjusted	8
Decrease in future depreciation expense		3,800,000
Increase in pre-tax income		945,000
Increase in net income		$ 945,000
Increase in basic earnings per share (in dollars per share)		$ 0.13
Increase in diluted earnings per share (in dollars per share)		$ 0.13