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REGULATORY MATTERS (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Net regulatory asset balance at beginning of period $ 29.9 $ 6.0 $ 7.7
Current year under recovery adjustments 11.9 29.9 6.0
Amortization of prior year regulatory asset (29.9) (6.0) (7.7)
Net regulatory asset balance at end of period 11.9 29.9 6.0
Allowance for equity used during construction $ 45.5 70.3 57.2
Southern Access Pipeline [Member]      
Amortization through revenue     6.1
Alberta Clipper Pipeline [Member]      
Obligation to refund overcollection to customers (in percentage) 50.00%    
Amortization through revenue recovery assets $ 0.8    
Amortization through revenue   5.9 $ 6.9
Recovery assets $ 0.1 $ 0.8