Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Trade receivables, allowance for doubtful accounts	$ 5,540	$ 5,610
Percentage of cumulative preferred stock	4.50%	4.50%
Preferred stock, par value (dollars per share)	$ 50	$ 50
Common stock, par value (dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	180,000,000	180,000,000
Common stock, shares issued (in shares)	62,081,391	62,081,391
Common stock, shares outstanding (in shares)	38,222,928	37,736,578