Consolidated Balance Sheets (Parentheticals) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
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)	27,994,000	27,361,000
Common stock, shares outstanding (in shares)	25,792,000	25,114,000
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
Common stock, shares outstanding (in shares)	0
Allowance for doubtful accounts of trade accounts receivable	$ 1,480,000	$ 2,506,000
Accumulated depreciation of vessels and equipment	457,670,000	428,538,000
Accumulated amortization	$ 0	$ 18,741,000
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	2,000,000	2,000,000
Preferred stock, shares issued (in shares)	0	0