INCOME TAXES - Schedule of Components of Deferred Income Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Total deferred income tax assets	$ 65.8	$ 2.0
Total deferred income tax liabilities	341.9	477.8
Total deferred income taxes:
Property, plant and equipment	317.1	448.2
2017 Tax Reform Act impact	(60.7)
Employee benefits	19.3	27.9
Deferred compensation	(1.4)	(0.6)
Purchased gas costs	1.5	0.1
Other	0.3	0.2
Total net deferred income tax liabilities	$ 276.1	$ 475.8