PENSION AND POST-RETIREMENT PLANS - AMOUNTS INCLUDED IN FINANCIAL STATEMENTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee benefit plans
Current liabilities	$ (2.7)	$ (3.1)
Non-current liabilities, net	(82.0)	(73.4)
Pension Benefits
Employee benefit plans
Projected benefit obligations	263.1	239.9	$ 217.4
Fair value of plan assets at end of year	212.8	194.8	162.2
Pension Benefits | Qualified Plan
Employee benefit plans
Non-current assets	1.0	1.0
Non-current liabilities, net	(51.3)	(46.1)
Total	(50.3)	(45.1)
Net loss, net of taxes	(64.2)	(56.2)
Prior service credit, net of taxes	2.5	4.3
Total	(61.7)	(51.9)
Projected benefit obligations	261.4	238.3
Accumulated benefit obligation	260.9	237.9
Fair value of plan assets at end of year	210.0	192.2
Pension Benefits | Nonqualified Plan
Employee benefit plans
Current liabilities	(1.7)	(2.1)
Non-current liabilities, net	(2.6)	(2.3)
Total	(4.3)	(4.4)
Net loss, net of taxes	(0.8)	(0.6)
Prior service credit, net of taxes	0.1	0.2
Total	(0.7)	(0.4)
Post-retirement Benefits
Employee benefit plans
Projected benefit obligations	$ 29.1	26.0	$ 22.2
Amortization period of unrecognized gains and losses	5 years
Post-retirement Benefits | Qualified Plan
Employee benefit plans
Current liabilities	$ (1.0)	(1.0)
Non-current liabilities, net	(28.1)	(25.0)
Total	(29.1)	(26.0)
Net loss, net of taxes	(4.4)	(2.8)
Prior service credit, net of taxes	16.6	19.1
Total	$ 12.2	$ 16.3