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REGULATORY MATTERS (Tables)
 9 Months Ended
Sep. 30, 2017
Regulatory Assets and Liabilities Disclosure [Abstract]  
Schedule of Changes in Net Regulatory Asset Balance
Our over and under recovery revenue adjustments and net regulatory asset amortization for the three and nine months ended September 30, 2017 and 2016 are as follows:
 
Three months ended September 30,
 
Nine months ended September 30,
  
2017
 
2016
 
2017
 
2016
 
 
 
 
 
 
 
 
  
(in millions)
Net regulatory asset (liability) balance at beginning of period
$
(23.7
)
 
$
29.3

 
$
11.9

 
$
29.9

Current period under recovery revenue adjustments
28.8

 
6.4

 
1.8

 
20.0

Amortization of prior year regulatory asset
(1.6
)
 
(7.9
)
 
(10.2
)
 
(22.1
)
Net regulatory asset balance at end of period
$
3.5

 
$
27.8

 
$
3.5

 
$
27.8