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ASSET RETIREMENT OBLIGATIONS (Tables)
 12 Months Ended
Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Asset Retirement Obligations
The following is a reconciliation of the beginning and ending aggregate carrying amounts of our ARO liabilities for each of the years ended December 31, 2016 and 2015:
 
 
 
2016
 
2015
 
 
 
 
 
 
 
 
 
 
 
(in millions)
 
Balance at beginning of period
 
$
103.6
 
$
96.8
 
Revisions in estimates
 
 
(1.0)
 
 
14.1
 
Accretion expense
 
 
4.6
 
 
1.5
 
Liabilities settled
 
 
(6.1)
 
 
(8.8)
 
Balance at end of period
 
$
101.1
 
$
103.6