PENSION AND POST-RETIREMENT PLANS - AMOUNTS INCLUDED IN FINANCIAL STATEMENTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee benefit plans
Current liabilities	$ (3.0)	$ (2.1)
Non-current liabilities, net	(75.1)	(75.0)
Pension Benefits
Employee benefit plans
Benefit obligation	232.1	225.4	$ 220.2
Fair value of plan assets at end of year	186.7	178.8	168.9
Pension Benefits | Qualified Plan
Employee benefit plans
Non-current assets	0.5	1.0
Non-current liabilities, net	(45.9)	(47.6)
Total	(45.4)	(46.6)
Net loss, net of taxes	(46.9)	(49.1)
Prior service credit (net of taxes)	6.3	7.7
Total	(40.6)	(41.4)
Benefit obligation	229.9	223.2
Accumulated benefit obligation	229.6	222.9
Fair value of plan assets at end of year	184.7	175.9
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2018	1.4
Pension Benefits | Nonqualified Plan
Employee benefit plans
Current liabilities	(1.8)	(1.0)
Non-current liabilities, net	(2.7)	(3.3)
Total	(4.5)	(4.3)
Net loss, net of taxes	(0.6)	(0.8)
Prior service credit (net of taxes)	0.3	0.4
Total	(0.3)	(0.4)
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2018	0.1
Other Post-retirement Benefits
Employee benefit plans
Benefit obligation	27.7	25.2	$ 60.5
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2018	$ 1.3
Amortization period of unrecognized gains and losses	5 years
Other Post-retirement Benefits | Qualified Plan
Employee benefit plans
Current liabilities	$ (1.2)	(1.1)
Non-current liabilities, net	(26.5)	(24.1)
Total	(27.7)	(25.2)
Net loss, net of taxes	(4.6)	(4.4)
Prior service credit (net of taxes)	20.2	22.5
Total	$ 15.6	$ 18.1