Changes In Accounting Estimates (Details) (USD $) In Millions, unless otherwise specified		3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2013	Mar. 31, 2013 Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member] item	Dec. 31, 2013 Extended Economic Life on International Flag Special Purpose Roll-On/Roll-Off [Member]	Dec. 31, 2013 Extended Economic Life of Both Mobile Alabama and Coatzacoalcos [Member]	Mar. 31, 2012 Change In Salvage Value [Member] item	Dec. 31, 2013 Change In Salvage Value [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
Average remaining useful lives of vessels						9 years
Decrease in future depreciation expense			$ 1.1			$ 3.8
Increase in pre-tax income			1.1	1.1		3.8
Increase in net income			$ 1.1	$ 1.1		$ 3.7
Number of vessels to have extended life		2
Extended amortization period of assets				5 years