Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Allowance for doubtful accounts receivable	$ 27,938	$ 24,287
Property and equipment, accumulated depreciation	410,243	373,180
Acquired intangible assets, accumulated amortization	204,853	190,920
Other assets, accumulated amortization	$ 46,788	$ 50,821
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	62,775,761	59,897,309
Treasury stock, shares	8,554,908	8,077,311