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MATERIALS, SUPPLIES, AND INVENTORIES (Tables)
6 Months Ended
Jun. 30, 2017
Inventory Disclosure [Abstract]  
Schedule of inventory
Our inventory consisted of:
(in millions)
 
June 30, 2017
 
December 31, 2016
Natural gas in storage
 
$
137.8

 
$
223.1

Materials and supplies
 
216.3

 
206.5

Fossil fuel
 
155.9

 
158.0

Total
 
$
510.0

 
$
587.6