PENSION AND POST-RETIREMENT PLANS - AMOUNTS INCLUDED IN FINANCIAL STATEMENTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee benefit plans
Current liabilities	$ (3.1)	$ (3.0)
Non-current liabilities, net	(73.4)	(79.4)
Pension Benefits
Employee benefit plans
Benefit obligation	239.9	217.4	$ 232.1
Fair value of plan assets at end of year	194.8	162.2	186.7
Pension Benefits | Qualified Plan
Employee benefit plans
Non-current assets	1.0	0.8
Non-current liabilities, net	(46.1)	(56.0)
Total	(45.1)	(55.2)
Net loss, net of taxes	(56.2)	(61.8)
Prior service credit, net of taxes	4.3	6.0
Total	(51.9)	(55.8)
Benefit obligation	238.3	215.9
Accumulated benefit obligation	237.9	215.6
Fair value of plan assets at end of year	192.2	159.9
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2020	1.5
Pension Benefits | Nonqualified Plan
Employee benefit plans
Current liabilities	(2.1)	(1.8)
Non-current liabilities, net	(2.3)	(2.4)
Total	(4.4)	(4.2)
Net loss, net of taxes	(0.6)	(0.4)
Prior service credit, net of taxes	0.2	0.3
Total	(0.4)	(0.1)
Post-retirement Benefits
Employee benefit plans
Benefit obligation	26.0	22.2	$ 27.7
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2020	$ 2.3
Amortization period of unrecognized gains and losses	5 years
Post-retirement Benefits | Qualified Plan
Employee benefit plans
Current liabilities	$ (1.0)	(1.2)
Non-current liabilities, net	(25.0)	(21.0)
Total	(26.0)	(22.2)
Net loss, net of taxes	(2.8)	(0.1)
Prior service credit, net of taxes	19.1	21.8
Total	$ 16.3	$ 21.7