PENSION AND POST-RETIREMENT PLANS - COMPONENTS OF NET PERIODIC BENEFIT COST (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Net Periodic Benefit Cost (Benefit):
Current liabilities	$ 3.1	$ 3.0
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
New prior service cost (credit)			$ (0.8)
Amortization of net loss	(2.7)	(1.1)	(1.7)
Amortization of prior service cost	$ 4.5	$ 4.7	4.0
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Discount rate used to determine obligation (as a percent)	2.80%	4.00%
Pension Benefits
Components of Net Periodic Benefit Cost (Benefit):
Service cost	$ 4.7	$ 4.4	4.0
Interest cost	9.3	8.6	9.7
Expected return on plan assets	(11.9)	(13.5)	(13.5)
Amortization of net loss	5.2	4.6	5.1
Amortization of prior service credit	(2.3)	(2.3)	(2.3)
Net periodic benefit cost (benefit)	5.0	1.8	3.0
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Net loss (gain)	(1.7)	7.8	0.8
Amortization of net loss	(3.9)	(3.5)	(3.1)
Amortization of prior service cost	1.7	1.7	1.4
Total recognized in other comprehensive (income) loss	(3.9)	6.0	(0.9)
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 1.1	$ 7.8	$ 2.1
Weighted Average Assumptions:
Discount rate (as a percent)	3.40%	4.40%	3.80%
Expected return plan assets (as a percent)	7.50%	7.50%	7.75%
Rate of compensation increase (as a percent)	3.00%	3.00%	3.00%
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	$ 239.9	$ 217.4	$ 232.1
Post-retirement Benefits
Components of Net Periodic Benefit Cost (Benefit):
Service cost	0.4	0.6	0.5
Interest cost	0.9	1.0	1.1
Amortization of net loss	(0.1)	1.5	1.2
Amortization of prior service credit	(3.8)	(3.8)	(3.8)
Net periodic benefit cost (benefit)	(2.6)	(0.7)	(1.0)
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income, net of tax:
Net loss (gain)	2.5	(4.7)	1.1
Amortization of net loss	0.2	(1.1)	(0.7)
Amortization of prior service cost	2.8	2.8	2.3
Total recognized in other comprehensive (income) loss	5.5	(3.0)	2.7
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ 2.9	$ (3.7)	$ 1.7
Weighted Average Assumptions:
Discount rate (as a percent)	3.50%	4.50%	3.90%
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)	5.70%	6.00%	6.30%
Initial health care cost trend rate, Post-65 group (as a percent)	5.90%	6.30%	6.80%
Ultimate health care cost trend rate (as a percent)	4.40%	4.40%	4.40%
Effect of one percentage point change in the assumed health care cost trend rates
Effect of one percentage point increase on total of service and interest cost components	$ 0.2	$ 0.3	$ 0.3
Effect of one percentage point increase on post-retirement benefit obligation	3.3	2.6	4.0
Effect of one percentage point decrease on total of service and interest cost components	(0.2)	(0.2)	(0.2)
Effect of one percentage point decrease on post-retirement benefit obligation	(2.6)	(2.0)	(3.0)
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total 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
Qualified Plan | Pension Benefits
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	238.3	215.9
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(56.2)	(61.8)
Amount recognized as prior service credit, net of tax	4.3	6.0
Total	(51.9)	(55.8)
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
Qualified Plan | Post-retirement Benefits
Components of Net Periodic Benefit Cost (Benefit):
Current liabilities	1.0	1.2
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(2.8)	(0.1)
Amount recognized as prior service credit, net of tax	19.1	21.8
Total	16.3	21.7
Nonqualified Plan | Pension Benefits
Components of Net Periodic Benefit Cost (Benefit):
Current liabilities	2.1	1.8
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(0.6)	(0.4)
Amount recognized as prior service credit, net of tax	0.2	0.3
Total	$ (0.4)	$ (0.1)