Unaudited Condensed Consolidated Balance Sheets (Parentheticals) (USD $) In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Allowance for doubtful accounts of trade accounts receivable (in Dollars)	$ 2,624	$ 3,250
Accumulated depreciation for vessels and equipment (in Dollars)	400,035	389,469
Accumulated amortization of intangibles (in Dollars)	$ 14,417	$ 12,975
Preferred stock, shares authorized (in Shares)	2,000	2,000
Preferred stock, shares issued (in Shares)	0	0
Preferred stock, par value (in Dollars per share)	$ 0	$ 0
Common Class A [Member]
Common Stock, par value (in Dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in Shares)	60,000	60,000
No. of common shares issued (in Shares)	27,153	26,941
Common Stock, shares outstanding (in Shares)	26,796	26,906
Common Stock, shares authorized (in Shares)	60,000	60,000
No. of common shares issued (in Shares)	27,153	26,941
Common Class B [Member]
Common Stock, par value (in Dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in Shares)	60,000	60,000
No. of common shares issued (in Shares)	0	0
Common Stock, shares authorized (in Shares)	60,000	60,000
No. of common shares issued (in Shares)	0	0