Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowances	$ 19,558	$ 16,280
Property and equipment, accumulated depreciation	$ 259,730	$ 239,581
Common stock, par value (in USD per share)	$ 2	$ 2
Common stock, authorized (in shares)	200,000,000	200,000,000
Common stock, issued (in shares)	62,269,000	61,476,000
Common stock, outstanding (in shares)	62,269,000	61,476,000