Condensed Consolidated Balance Sheets - Parenthetical - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 19,173	$ 18,228
Common Stock, Par value per share (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares authorized	300,000,000	300,000,000
Common Stock, Shares outstanding	79,457,531	79,038,062
Treasury stock, at cost (shares)	399,385	402,342