Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 46,031	$ 32,506
Par value (in usd per share)	$ 0.001	$ 0.001
Shares authorized (in shares)	475,000,000	475,000,000
Shares issued (in shares)	122,083,059	121,259,960
Shares outstanding (in shares)	89,803,982	91,836,938
Treasury stock, shares	32,279,077	29,423,022