Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $) In Thousands, except Per Share data	Jun. 30, 2011	Dec. 31, 2010
Current assets:
Allowance for doubtful accounts of trade accounts receivable	$ 97	$ 283
Accumulated depreciation for vessels and equipment	318,403	282,395
Accumulated amortization of intangibles	$ 8,650	$ 7,208
Stockholders' equity:
Preferred stock, no par value	$ 0	$ 0
Preferred stock, shares authorized	2,000	2,000
Preferred stock, shares issued	0	0
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	26,540	26,269
Common stock, shares outstanding	26,513	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	0	0