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ASSET RETIREMENT OBLIGATION: (Tables)
 6 Months Ended
Jun. 30, 2017
ASSET RETIREMENT OBLIGATION:  
Summary of asset retirement obligation activity

 

The following table summarizes the asset retirement obligation activity for the six months ended June 30, 2017 and 2016 (in millions):

 

 

 

2017

 

2016

 

Balance as of January 1

 

$
216.5

 

$
190.9

 

Changes in estimates

 

—

 

—

 

Payments

 
(0.3

)
(1.4

)

Accretion expense

 
6.6

 
13.9

 

 

 

 

 

 

 

Balance as of June 30,

 

$
222.8

 

$
203.4