CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 2,323	$ 3,191
Long-term debt, debt issuance cost	$ 1,217	$ 1,563
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	$ 0.01	$ 0.01
Common stock, authorized	300,000,000	300,000,000
Common stock, issued	266,259,083	191,525,872
Common stock, outstanding	222,456,472	148,133,255
Treasury stock, shares	43,802,611	43,392,617