PENSION AND POST-RETIREMENT PLANS - RETURN ON PLAN ASSETS (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in plan assets:
Balance at the beginning of the period	$ 178.8	$ 168.9
Actual return (loss) on plan assets:
Balance at the end of the period	186.7	178.8
Significant Unobservable Inputs (Level 3) | Private equity partnership interests
Changes in plan assets:
Balance at the beginning of the period	0.1	0.2
Actual return (loss) on plan assets:
Assets held at the reporting date	0.1	(0.1)
Assets sold during the period	(0.1)	0.1
Purchases, sales and settlements, net		(0.1)
Balance at the end of the period	0.1	0.1
Fair Value Measurements Recurring | Significant Unobservable Inputs (Level 3)
Changes in plan assets:
Balance at the beginning of the period	10.9	10.5
Actual return (loss) on plan assets:
Assets held at the reporting date	0.4	0.5
Assets sold during the period	0.4	0.6
Purchases, sales and settlements, net	(0.5)	(0.7)
Balance at the end of the period	11.2	10.9
Fair Value Measurements Recurring | Significant Unobservable Inputs (Level 3) | Real estate partnerships interests
Changes in plan assets:
Balance at the beginning of the period	10.8	10.3
Actual return (loss) on plan assets:
Assets held at the reporting date	0.3	0.6
Assets sold during the period	0.5	0.5
Purchases, sales and settlements, net	(0.5)	(0.6)
Balance at the end of the period	$ 11.1	$ 10.8