Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 66,194	$ 59,963
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	475,000,000	475,000,000
Common stock, shares issued (in shares)	123,406,538	123,035,859
Common stock, shares outstanding (in shares)	86,189,402	85,845,344
Treasury stock, shares (in shares)	37,217,136	37,190,515