CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 32,670	$ 31,266
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	349,000,000	349,000,000
Common stock, shares issued	267,997,000	266,545,000
Common stock, shares outstanding	180,510,000	189,308,000
Treasury stock at cost, shares	87,487,000	77,237,000