CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 2,312	$ 2,323
Long-term debt, debt discount and issuance costs	$ 5,998	$ 1,217
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, authorized	1,000,000	1,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$ 1	$ 1
Common stock, authorized	400,000,000	300,000,000
Common stock, issued	267,089,563	266,259,083
Common stock, outstanding	217,209,292	222,456,472
Treasury stock, shares	49,880,271	43,802,611