Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts receivable	$ 24,287	$ 20,958
Property and equipment, accumulated depreciation	373,180	340,128
Acquired intangible assets, accumulated amortization	190,920	179,142
Other assets, accumulated amortization	$ 50,821	$ 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	59,897,309	58,892,744
Treasury stock, shares	8,077,311	6,084,586