Balance Sheet (Parenthetical) (USD $) In Thousands, except Share data	Jun. 30, 2011	Dec. 31, 2010
Allowance for doubtful accounts	$ 5,108	$ 5,026
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	58,353,562	56,518,417
Treasury stock, at cost	2,723,337	2,691,215