CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current Assets:
Allowance for doubtful accounts on trade accounts receivable	$ 18,967	$ 16,966
Common stock, par value (in dollars per share)	$ 86.08
Class A common stock
Current Assets:
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	1,600,000,000	1,600,000,000
Common stock, shares issued	229,642,174	229,448,857
Common stock, shares outstanding	229,642,174	229,448,857
Class B common stock
Current Assets:
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	800,000,000	800,000,000
Common stock, shares issued	238,435,208	238,435,208
Common stock, shares outstanding	238,435,208	238,435,208