Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 48,245	$ 46,031
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)	122,551,794	122,083,059
Common stock, shares outstanding (in shares)	88,376,611	89,803,982
Treasury stock, shares (in shares)	34,175,183	32,279,077