FAIR VALUE MEASUREMENTS - LEVEL 3 RECONCILIATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Level 3 Rollforward
Balance at the beginning of period	$ 2.0	$ 2.0	$ (0.3)
Realized and unrealized losses	0.0	(0.2)	(10.7)
Purchases	6.9	7.1	9.8
Sales	0.0	(0.2)	(0.1)
Settlements	(6.9)	(6.7)	(1.4)
Transfers out of Level 3	0.0	0.0	4.7
Balance at the end of the period	2.0	2.0	2.0
Unrealized gains and losses on level 3 derivatives included in earnings	$ 0.0	$ 0.0	$ 0.0