PENSION AND POST-RETIREMENT PLANS - COMPONENTS OF NET PERIODIC BENEFIT COST (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Net Periodic Benefit Costs:
Current liabilities	$ 3.0	$ 2.1
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (net of tax)
New prior service cost	(0.8)	(24.1)	$ (5.1)
Amortization of net loss	(1.7)	(1.2)	(1.8)
Amortization of prior service cost	$ 4.0	$ 2.2	1.3
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Discount rate used to determine obligation (as a percent)	3.20%	3.40%
Pension Benefits
Components of Net Periodic Benefit Costs:
Service cost	$ 4.0	$ 3.9	3.3
Interest cost	9.7	9.7	9.5
Expected return on plan assets	(13.5)	(13.4)	(14.0)
Amortization of net loss	5.1	5.5	6.4
Amortization of prior service cost	(2.3)	(2.3)	(2.3)
Net periodic benefit cost	3.0	3.4	2.9
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (net of tax)
Net loss (gain)	0.8	1.6	(1.0)
New prior service cost			0.1
Amortization of net loss	(3.1)	(3.3)	(3.9)
Amortization of prior service cost	1.4	1.4	1.4
Total recognized in other comprehensive income	(0.9)	(0.3)	(3.4)
Total recognized in net periodic benefit cost and other comprehensive income	$ 2.1	$ 3.1	$ (0.5)
Weighted Average Assumptions:
Discount rate (as a percent)	3.80%	4.40%	4.50%
Expected return plan assets (as a percent)	7.75%	8.00%	8.00%
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	$ 232.1	$ 225.4	$ 220.2
Other Post-retirement Benefits
Components of Net Periodic Benefit Costs:
Service cost	0.5	1.5	1.5
Interest cost	1.1	2.7	2.5
Amortization of net loss	1.2	1.2	2.2
Amortization of prior service cost	(3.8)	(1.4)
Net periodic benefit cost	(1.0)	4.0	6.2
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (net of tax)
Net loss (gain)	1.1	1.2	(1.9)
New prior service cost		(23.4)
Amortization of net loss	(0.7)	(0.8)	(1.3)
Amortization of prior service cost	2.3	0.9
Total recognized in other comprehensive income	2.7	(22.1)	(3.2)
Total recognized in net periodic benefit cost and other comprehensive income	$ 1.7	$ (18.1)	$ 3.0
Weighted Average Assumptions:
Discount rate (as a percent)	3.90%	4.60%	4.60%
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)	6.30%	6.60%	6.80%
Initial health care cost trend rate, Post-65 group (as a percent)	6.80%	7.20%	7.60%
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	$ 0.3	$ 0.9	$ 0.9
Effect of one percentage point increase on post-retirement benefit obligation	4.0	11.5	9.4
Effect of one percentage point decrease on total of service and interest cost components	(0.2)	(0.7)	(0.7)
Effect of one percentage point decrease on post-retirement benefit obligation	(3.0)	(8.3)	(7.4)
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	27.7	25.2	$ 60.5
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2018	1.3
Qualified Plan | Pension Benefits
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	229.9	223.2
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(46.9)	(49.1)
Amount recognized as prior service credit, net of tax	6.3	7.7
Total	(40.6)	(41.4)
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2018	1.4
Qualified Plan | Other Post-retirement Benefits
Components of Net Periodic Benefit Costs:
Current liabilities	1.2	1.1
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	(4.6)	(4.4)
Amount recognized as prior service credit, net of tax	20.2	22.5
Total	15.6	18.1
Nonqualified Plan | Pension Benefits
Components of Net Periodic Benefit Costs:
Current liabilities	1.8	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	(0.6)	(0.8)
Amount recognized as prior service credit, net of tax	0.3	0.4
Total	(0.3)	$ (0.4)
Estimated net loss and prior service credit that will be amortized from accumulated other comprehensive loss into net periodic benefit cost in 2018	$ 0.1