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Product Warranties (Tables)
6 Months Ended
Jun. 30, 2011
Product Warranties [Abstract]  
Company's accrual for product warranty claims
                 
(Thousands of dollars)   2011     2010  
Balance at January 1
  $ 23,103     $ 22,214  
Charges to expense
    14,445       10,869  
Settlements
    (12,746 )     (10,478 )
 
           
Balance at June 30
  $ 24,802     $ 22,605