Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Allowance for doubtful accounts of trade accounts receivable	$ 223	$ 198
Accumulated depreciation for vessels and equipment	350,081	329,299
Accumulated amortization of intangibles	$ 10,751	$ 10,030
Preferred stock, no par value
Preferred stock, shares authorized	2,000	2,000
Preferred stock, shares issued
Class A Common stock
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	60,000	60,000
No. of common shares issued	26,803	26,631
Common stock, shares outstanding	26,803	26,617
Class B Common stock
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	60,000	60,000
No. of common shares issued