ASSET RETIREMENT OBLIGATIONS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Balance at beginning of period	$ 103.6	$ 96.8
Revisions in estimates	(1.0)	14.1
Accretion expense	4.6	1.5
Liabilities settled	(6.1)	(8.8)
Balance at end of period	$ 101.1	$ 103.6