Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, accumulated provision for doubtful accounts	$ 8,235	$ 8,226
Common stock, par value (in dollars per share)	$ 0	$ 0
Common stock, authorized shares (in shares)	320,000,000	320,000,000
Common stock, shares issued (in shares)	257,225,000	267,871,000
Treasury stock, shares at cost (in shares)	2,799,000	3,551,000
Treasury stock, shares held in rabbi trust (in shares)	299,000	253,000