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Property, Plant and Equipment
9 Months Ended
Sep. 30, 2012
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment
(5)
Property, Plant and Equipment:
Property, plant and equipment, net is as follows:

($ in thousands)
 
September 30, 2012
   
December 31, 2011
   
                 
Property, plant and equipment
 
$
14,133,935
   
$
13,638,136
   
Less: Accumulated depreciation
   
(6,651,035
)
   
(6,090,613
)
 
Property, plant and equipment, net
 
$
7,482,900
   
$
7,547,523
   
                   

Depreciation expense is principally based on the composite group method. Depreciation expense was $209.9 million and $226.7 million, and $628.8 million and $671.2 million for the three and nine months ended September 30, 2012 and 2011, 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, 2011.