Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 59,963	$ 46,031
Par value (in usd per share)	$ 0.001	$ 0.001
Shares authorized (in shares)	475,000,000	475,000,000
Shares issued (in shares)	123,035,859	122,083,059
Shares outstanding (in shares)	85,845,344	89,803,982
Treasury stock, shares	37,190,515	32,279,077

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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