Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 16,275	$ 15,950
Common stock, par value per share (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Common stock, shares issued (in shares)	85,213,952	81,119,117
Common stock, shares outstanding (in shares)	84,819,158	80,722,258
Treasury stock, at cost (shares)	394,794	396,859