Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 18,228	$ 15,675
Preferred Stock, Par value per share (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares authorized	100,000,000	100,000,000
Preferred Stock, Shares issued	0	0
Preferred Stock, Shares outstanding	0	0
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 issued	79,440,404	71,817,430
Common Stock, Shares outstanding	79,038,062	71,409,015
Treasury stock, at cost (shares)	402,342	408,415