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Summary of Significant Accounting Policies (Tables)
6 Months Ended
Jun. 30, 2012
Accounting Policies [Abstract]  
Inventory Components
Inventory is recorded at actual cost when purchased and then expensed at weighted-average cost as used. Inventory consisted of the following:
(in millions)
June 30, 2012
 
December 31, 2011
Coal, fuel oil and other raw materials
$
142

 
$
188

Spare parts, materials and supplies
90

 
86

Total inventory
$
232

 
$
274