Current Period Unaudited Condensed Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
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)	27,759,000	27,994,000
Common Stock, shares outstanding (in shares)	26,830,000	25,792,000
Common Class B [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)	0	0
Allowance for doubtful accounts of trade accounts receivable	$ 1,360	$ 1,480
Accumulated depreciation of vessels and equipment	$ 468,613	$ 457,670
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)	$ 0.01	$ 0.01