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Level 4 Summary of Significant Accounting Policies (Details) (USD $)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Vendor rebates and allowances $ 4,000,000   $ 4,000,000   $ 4,100,000 $ 2,800,000  
Customer rebates payable 21,200,000   21,200,000   19,700,000 15,600,000  
Rental expense 3,700,000 3,600,000 7,300,000 7,200,000 14,300,000 14,500,000 14,200,000
Money market funds, fair value 176,800,000   176,800,000   10,600,000 164,600,000  
Concentration of credit risk         11.00% 10.00%  
Accounts Receivable [Member] | Credit Concentration Risk [Member]
              
Concentration of credit risk     16.00%   14.00% 14.00%  
6.375% senior notes [Member]
              
6.375% senior notes, book value 250,000,000   250,000,000   250,000,000 0  
6.375% senior notes, fair value $ 253,800,000   $ 253,800,000   $ 251,600,000