Unaudited Condensed Consolidated Balance Sheets (Parentheticals) - USD ($) shares in Thousands, $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Allowance for doubtful accounts of trade accounts receivable	$ 3,347	$ 2,482
Accumulated depreciation of vessels, equipment, and other fixed assets	$ 498,847	$ 468,817
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	2,000	2,000
Preferred stock, shares issued (in shares)	0	0
Common Class A [Member]
Common stock, shares authorized (in shares)	60,000	60,000
Common stock, shares issued (in shares)	29,629	29,104
Common stock, shares outstanding (in shares)	27,957	27,122
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Class B [Member]
Common stock, shares authorized (in shares)	60,000	60,000
Common stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01