CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts receivable, allowance	$ 30,696	$ 25,278
Treasury stock - Common stock, Shares	89,528,000	87,077,000
Class B Common Stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, Shares authorized	400,000,000	400,000,000
Common stock, Shares issued	12,800,000	12,800,000
Common stock, Shares outstanding	12,800,000	12,800,000
Common stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, Shares authorized	1,600,000,000	1,600,000,000
Common stock, Shares issued	228,467,000	224,310,000
Common stock, Shares outstanding	138,939,000	137,232,000