CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Accounts receivable, allowance	$ 39,214	$ 50,406
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	272,013,000	270,955,000
Common stock, shares outstanding	166,134,000	171,224,000
Treasury stock at cost, shares	105,879,000	99,731,000