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Fair Value of Financial Instruments (Tables)
3 Months Ended
Mar. 31, 2013
Fair Value of Financial Instruments [Abstract]  
Fair Value of Long-Term Debt
The fair value of our long-term debt is estimated based upon quoted market prices at the reporting date for those financial instruments.

 
March 31, 2013
 
December 31, 2012
 
Carrying
     
Carrying
   
($ in thousands)
Amount
 
Fair Value
 
Amount
 
Fair Value
               
Long-term debt
$  8,368,729
 
$  8,947,496
 
$  8,381,947
 
$  9,091,416