Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts receivable, allowances for doubtful accounts	$ 6,217	$ 8,288
Common stock, par value (in dollars per share)	$ 0.25	$ 0.25
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	110,834,088	110,834,088
Treasury stock, shares	12,554,714	12,768,726