Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 15,345	$ 12,858
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	171,582	176,374
Common stock, outstanding	171,582	176,374