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Property, Plant and Equipment
3 Months Ended
Mar. 31, 2013
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment
(5)Property, Plant and Equipment:
Property, plant and equipment, net is as follows:

   ($ in thousands)
 
March 31, 2013
   
December 31, 2012
   
    
                 
    Property, plant and equipment
 
14,429,251
   
$
14,353,763
   
    Less:  Accumulated depreciation
   
(7,011,505
)
   
(6,848,867
)
 
         Property, plant and equipment, net
 
$
7,417,746
   
$
7,504,896
   
                   

Depreciation expense is principally based on the composite group method.  Depreciation expense was $216.7 million and $210.3 million for the three months ended March 31, 2013 and 2012, respectively.  As a result of an independent study of the estimated remaining useful lives of our plant assets, we adopted new estimated remaining useful lives for certain plant assets as of October 1, 2012, with an immaterial impact to depreciation expense.