PENSION AND POST-RETIREMENT PLANS - COMPONENTS OF NET PERIODIC BENEFIT COST (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of Net Periodic Benefit Cost (Benefit):
Current liabilities	$ 2.7	$ 3.1
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Amortization of net loss	9.4	(2.7)	$ (1.1)
Amortization of prior service cost	$ 4.7	$ 4.5	4.7
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Discount rate used to determine obligation (as a percent)	1.80%	2.80%
Pension Benefits
Components of Net Periodic Benefit Cost (Benefit):
Service cost	$ 5.1	$ 4.7	4.4
Interest cost	7.9	9.3	8.6
Expected return on plan assets	(14.0)	(11.9)	(13.5)
Amortization of net loss	4.5	5.2	4.6
Amortization of prior service credit	(2.3)	(2.3)	(2.3)
Net periodic benefit cost (benefit)	1.2	5.0	1.8
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Net loss (gain)	11.4	(1.7)	7.8
Amortization of net loss	(3.4)	(3.9)	(3.5)
Amortization of prior service cost	1.7	1.7	1.7
Total recognized in other comprehensive (income) loss	9.7	(3.9)	6.0
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 10.9	$ 1.1	$ 7.8
Weighted Average Assumptions:
Discount rate (as a percent)	2.50%	3.40%	4.40%
Expected return plan assets (as a percent)	7.25%	7.50%	7.50%
Rate of compensation increase (as a percent)	3.00%	3.00%	3.00%
Cash balance interest credit rate			3.01%
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	$ 263.1	$ 239.9	$ 217.4
Pension Benefits | Minimum
Weighted Average Assumptions:
Cash balance interest credit rate	0.75%	1.75%
Pension Benefits | Maximum
Weighted Average Assumptions:
Cash balance interest credit rate	3.25%	3.75%
Post-retirement Benefits
Components of Net Periodic Benefit Cost (Benefit):
Service cost	$ 0.5	$ 0.4	0.6
Interest cost	0.8	0.9	1.0
Amortization of net loss	0.5	(0.1)	1.5
Amortization of prior service credit	(3.7)	(3.8)	(3.8)
Net periodic benefit cost (benefit)	(1.9)	(2.6)	(0.7)
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Net loss (gain)	2.0	2.5	(4.7)
Amortization of net loss	(0.3)	0.2	(1.1)
Amortization of prior service cost	2.8	2.8	2.8
Total recognized in other comprehensive (income) loss	4.5	5.5	(3.0)
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 2.6	$ 2.9	$ (3.7)
Weighted Average Assumptions:
Discount rate (as a percent)	2.70%	3.50%	4.50%
Rate of compensation increase (as a percent)	3.00%	3.00%	3.00%
Initial health care cost trend rate, Pre-65 group (as a percent)	5.30%	5.70%	6.00%
Initial health care cost trend rate, Post-65 group (as a percent)	5.40%	5.90%	6.30%
Ultimate health care cost trend rate (as a percent)	4.40%	4.40%	4.40%
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	$ 29.1	$ 26.0	$ 22.2
Qualified Plan | Pension Benefits
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	261.4	238.3
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(64.2)	(56.2)
Amount recognized as prior service credit, net of tax	2.5	4.3
Total	(61.7)	(51.9)
Qualified Plan | Post-retirement Benefits
Components of Net Periodic Benefit Cost (Benefit):
Current liabilities	1.0	1.0
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(4.4)	(2.8)
Amount recognized as prior service credit, net of tax	16.6	19.1
Total	12.2	16.3
Nonqualified Plan | Pension Benefits
Components of Net Periodic Benefit Cost (Benefit):
Current liabilities	1.7	2.1
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(0.8)	(0.6)
Amount recognized as prior service credit, net of tax	0.1	0.2
Total	$ (0.7)	$ (0.4)