INCOME TAXES - Schedule of Components of Deferred Income Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Total deferred income tax assets	$ 2.0	$ 3.4
Total deferred income tax liabilities	477.8	440.1
Total deferred income taxes:
Property, plant and equipment	448.2	403.0
Employee benefits	27.9	28.0
Deferred compensation	(0.6)	(0.9)
Purchased gas costs	0.1	8.5
Other	0.2	(1.9)
Total deferred income tax liabilities	$ 475.8	$ 436.7