Consolidated Balance Sheets (Parentheticals) - USD ($) shares in Thousands, $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts of trade accounts receivable	$ 2,482	$ 1,480
Accumulated depreciation of vessels, equipment, and other fixed assets	$ 468,817	$ 457,670
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)	$ 0.01	$ 0.01
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
Common stock, shares issued (in shares)	29,104	27,994
Common stock, shares outstanding (in shares)	27,122	25,792
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
Common stock, shares issued (in shares)	0	0