Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 4,296	$ 4,022
Preferred Stock, par value	$ 0.01	$ 0.01
Preferred Stock, shares authorized	100,000,000	100,000,000
Preferred Stock, shares outstanding	0	0
Common Stock, par value	$ 0.01	$ 0.01
Common Stock, shares authorized	2,000,000,000	2,000,000,000
Common Stock, shares issued	132,478,073	131,750,715
Common Stock, shares outstanding	117,345,996	116,790,470
Treasury stock, shares	15,132,077	14,960,245