Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 12,858	$ 9,247
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, authorized	2,000	2,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized	640,000	640,000
Common stock, issued	176,374	179,857
Common stock, outstanding	176,374	179,857