Balance Sheet Parenthetical (Parentheticals) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Allowance for doubtful accounts receivable	$ 23,426	$ 20,958
Accumulated depreciation of property and equipment	365,708	340,128
Accumulated amortization of intangible assets	191,256	179,142
Accumulated amortization of other assets	$ 49,010	$ 44,469
Preferred Stock, par value per share	$ 0.02	$ 0.02
Preferred Stock, shares authorized	10,000,000	10,000,000
Preferred Stock, shares issued	0	0
Common Stock, par value per share	$ 0.02	$ 0.02
Common Stock, shares authorized	90,000,000	90,000,000
Common Stock, shares issued	59,554,968	58,892,744
Treasury Stock, shares	8,096,314	6,084,586