Unaudited Condensed Consolidated Balance Sheets (Parentheticals) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts of trade accounts receivable	$ 1,512	$ 2,506
Accumulated depreciation of vessels and equipment	433,027	428,538
Accumulated amortization of intangibles	$ 19,461	$ 18,741
Preferred stock, shares authorized (in shares)	2,000,000	2,000,000
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)	60,000,000	60,000,000
Common Stock, shares issued (in shares)	27,878,000	27,361,000
Common Stock, shares outstanding (in shares)	25,636,000	25,114,000
Common Class B [Member]
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	60,000,000	60,000,000
Common Stock, shares issued (in shares)	0	0