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ASSET RETIREMENT OBLIGATION: (Tables)
 9 Months Ended
Sep. 30, 2013
ASSET RETIREMENT OBLIGATION:  
Summary of the asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the nine months ended September 30, 2013 and 2012 (in millions):

 

 

 

2013

 

2012

 

 

 

 

 

 

 

Balance as of January 1

 

$

118.2

 

$

62.0

 

Closure payments

 

(0.7

)

—

 

Accretion expense

 

7.4

 

2.6

 

Balance as of September 30,

 

$

124.9

 

$

64.6