Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $) In Thousands, except Per Share data	Sep. 30, 2011	Dec. 31, 2010
Current assets:
Allowance for doubtful accounts of trade accounts receivable	$ 154	$ 283
Accumulated depreciation for vessels and equipment	320,945	282,395
Accumulated amortization of intangibles	$ 9,370	$ 7,208
Stockholders' equity:
Preferred stock, no par value
Preferred stock, shares authorized	2,000	2,000
Preferred stock, shares issued
Class A Common stock
Stockholders' equity:
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	60,000	60,000
Common stock, shares issued	26,553	26,269
Common stock, shares outstanding	26,533	26,013
Class B Common stock
Stockholders' equity:
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	60,000	60,000
Common stock, shares issued