CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 37,618	$ 32,670
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	269,116,000	267,997,000
Common stock, shares outstanding	180,612,000	180,510,000
Treasury stock at cost, shares	88,504,000	87,487,000