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Valuation and Qualifying Accounts (Details) (Allowance for Doubtful Accounts [Member], USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Allowance for Doubtful Accounts [Member]
     
Valuation and Qualifying Accounts      
Balance beginning of period $ 7,279 $ 7,935 $ 6,175
Additions charged to expense 23,728 24,877 24,216
Deductions (24,411) (25,533) (22,456)
Balance end of period $ 6,596 $ 7,279 $ 7,935
Bad debt expense as a % of revenue 4.20% 4.00% 3.80%