PENSION AND POST-RETIREMENT PLANS - COMPONENTS OF NET PERIODIC BENEFIT COST (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Total recognized in other comprehensive (income) loss	$ 2.5	$ (23.8)	$ (20.4)
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Discount rate used to determine obligation (as a percent)	5.20%	5.50%
Pension Benefits
Components of Net Periodic Benefit Cost (Credit):
Service cost	$ 3.1	$ 4.8	4.8
Interest cost	10.0	7.0	6.4
Expected return on plan assets	(13.9)	(16.0)	(14.7)
Amortization of net loss (gain)	1.3	2.4	5.2
Amortization of prior service credit		(1.0)	(2.3)
Net periodic benefit cost (credit)	0.5	(2.8)	(0.6)
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Net loss (gain)	(4.5)	(12.3)	(20.4)
Amortization of net loss (gain)	(1.0)	(1.8)	(3.9)
Amortization of prior service credit		0.8	1.7
Total recognized in other comprehensive (income) loss	(5.5)	(13.3)	(22.6)
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ (5.0)	$ (16.1)	$ (23.2)
Weighted Average Assumptions:
Discount rate (as a percent)	5.30%	5.60%	2.90%
Expected return plan assets (as a percent)	6.85%	6.75%	7.00%
Rate of compensation increase (as a percent)	3.50%		3.00%
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Benefit obligation at end of year	$ 193.1	$ 187.4	$ 249.5
Pension Benefits | Minimum
Weighted Average Assumptions:
Rate of compensation increase (as a percent)		3.50%
Cash balance interest credit rate	3.25%	3.25%	1.50%
Pension Benefits | Maximum
Weighted Average Assumptions:
Rate of compensation increase (as a percent)		4.00%
Cash balance interest credit rate	4.50%	3.50%	3.25%
Post-retirement benefits
Components of Net Periodic Benefit Cost (Credit):
Service cost	$ 0.2	$ 0.7	$ 0.7
Interest cost	0.8	0.8	0.7
Amortization of net loss (gain)	(2.0)	0.8	1.0
Amortization of prior service credit	(3.7)	(3.6)	(3.7)
Net periodic benefit cost (credit)	(4.7)	(1.3)	(1.3)
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Net loss (gain)	3.4	(10.7)
Amortization of net loss (gain)	1.5	(0.6)	(0.7)
Amortization of prior service credit	2.8	2.7	2.8
Total recognized in other comprehensive (income) loss	7.7	(8.6)	2.1
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 3.0	$ (9.9)	$ 0.8
Weighted Average Assumptions:
Discount rate (as a percent)	5.40%	5.50%	3.00%
Rate of compensation increase (as a percent)	3.50%		3.00%
Immediate health care cost trend rate, Pre-65 group (as a percent)	6.80%	6.60%	5.70%
Immediate health care cost trend rate, Post-65 group (as a percent)	7.10%	6.10%	5.80%
Ultimate health care cost trend rate (as a percent)	3.90%	4.00%	4.00%
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Benefit obligation at end of year	$ 19.5	$ 15.3	$ 29.3
Post-retirement benefits | Minimum
Weighted Average Assumptions:
Rate of compensation increase (as a percent)		3.50%
Post-retirement benefits | Maximum
Weighted Average Assumptions:
Rate of compensation increase (as a percent)		4.00%
Qualified Plan | Pension Benefits
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(20.3)	$ (25.8)
Total	(20.3)	(25.8)
Qualified Plan | Post-retirement benefits
Components of Net Periodic Benefit Cost (Credit):
Current liabilities	1.1	0.9
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	2.7	7.6
Amount recognized as prior service credit, net of tax	8.3	11.1
Total	11.0	18.7
Nonqualified Plan | Pension Benefits
Components of Net Periodic Benefit Cost (Credit):
Current liabilities	1.2	0.7
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(0.2)	0.1
Total	$ (0.2)	$ 0.1