Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accumulated depreciation	$ 252,611	$ 226,366
Stockholders' Equity, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	200,000	200,000
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	50,000,000	50,000,000
Common stock, shares issued (in shares)	29,141,467	28,162,497
Shares of common stock held in treasury at cost (in shares)	1,325,975	1,054,364