CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 61,926	$ 37,735
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	270,506,000	269,335,000
Common stock, shares outstanding	174,707,000	180,832,000
Treasury stock at cost, shares	96,099,000	88,504,000