CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts receivable, accumulated provision for doubtful accounts	$ 8,226	$ 6,923
Common stock, par value (in dollars per share)	$ 0	$ 0
Common stock, authorized (in shares)	320,000,000	320,000,000
Common stock, issued (in shares)	267,871,000	177,896,000
Treasury stock (in shares)	3,551,000	5,069,000
Treasury stock, held in rabbi trust (in shares)	253,000	226,000