PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Distribution	$ 2,605.0	$ 2,436.7
General and other	345.6	369.5
Plant under construction	91.1	76.8
Total property, plant and equipment	$ 3,041.7	$ 2,883.0