PENSION AND POST-RETIREMENT PLANS - COMPONENTS OF NET PERIODIC BENEFIT COST (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) director	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Components of Net Periodic Benefit Cost:
Current liabilities	$ 2.5	$ 2.5
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (net of tax)
New prior service cost	(5.1)	31.4	$ (18.7)
Amortization of unrecognized loss	(1.8)	(2.5)	(4.7)
Amortization of prior service cost	1.3	1.3	1.3
Qualified Pension Benefits
Components of Net Periodic Benefit Cost:
Service cost	3.3	3.3	2.9
Interest cost	9.5	9.4	8.6
Expected return on plan assets	(14.0)	(14.1)	(11.9)
Amortization of net loss	6.4	3.0	6.8
Amortization of prior service cost	(2.3)	(2.3)	(2.3)
Net periodic benefit cost	2.9	(0.7)	4.1
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (net of tax)
Net loss (gain)	(1.0)	25.4	(19.6)
New prior service cost	0.1
Amortization of unrecognized loss	(3.9)	(1.8)	(4.2)
Amortization of prior service cost	1.4	1.4	1.4
Total recognized in other comprehensive income	(3.4)	25.0	(22.4)
Total recognized in net periodic benefit cost and other comprehensive income	$ (0.5)	$ 24.3	$ (18.3)
Weighted Average Assumptions:
Discount rate (as a percent)	4.50%	4.10%	4.90%
Expected return plan assets (as a percent)	8.00%	8.25%	8.25%
Rate of compensation increase (as a percent)	3.00%	3.00%	3.00%
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)		$ 17.6
Projected benefit obligation change		17.6
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	$ 220.2	237.4	$ 197.5
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	50.8	55.5
Amount recognized as prior service credit, net of tax	(9.1)	(10.6)
Estimated net loss and prior service credit, net of tax, that will be recognized in net periodic pension cost	41.7	44.9
Post-retirement Benefits
Components of Net Periodic Benefit Cost:
Service cost	1.5	1.1
Interest cost	2.5	2.6
Current liabilities	$ 2.5	$ 2.5
Weighted Average Assumptions:
Discount rate (as a percent)	4.60%	4.20%	5.00%
Rate of compensation increase (as a percent)	3.00%	3.00%	3.00%
Initial health care cost trend rate (as a percent)		7.10%	7.30%
Initial health care cost trend rate, Pre-65 group (as a percent)	6.80%
Initial health care cost trend rate, Post-65 group (as a percent)	7.60%
Ultimate health care cost trend rate (as a percent)	4.40%	4.50%	4.50%
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)		$ 3.9
Projected benefit obligation change		3.9
Effect of one percentage point change in the assumed health care cost trend rates
Effect of one percentage point increase on post-retirement benefit obligation	$ 9.4	10.0	$ 7.1
Effect of one percentage point decrease on post-retirement benefit obligation	(7.4)	(7.8)	(5.7)
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Total obligation	60.5	62.6	52.1
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	4.7	7.2
Estimated net loss and prior service credit, net of tax, that will be recognized in net periodic pension cost	4.7	7.2
Non-qualified Pension Benefits
Components of Net Periodic Benefit Cost:
Amortization of net loss	0.1
Amortization of prior service cost	0.1
Net periodic benefit cost	0.6	0.6	0.6
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	0.9	0.7	0.6
Effect of one percentage point decrease on total of service and interest cost components	$ (0.7)	(0.5)	(0.5)
Qualified Pension Plans, Non-qualified Pension Plans and Post-retirement Plans
Number of outside directors covered under the frozen non-qualified pension plan | director	1
Total obligation	$ 4.3	5.4
Discount rate used to determine obligation (as a percent)	3.40%
Amounts recognized in accumulated other comprehensive income for net loss, net of tax	$ 0.8
Amount recognized as prior service credit, net of tax	0.6
Non-qualified pension benefits and post-retirement benefits
Components of Net Periodic Benefit Cost:
Service cost	1.5	1.1	1.1
Interest cost	2.5	2.6	2.1
Amortization of net loss	2.2	0.6	0.3
Net periodic benefit cost	6.2	4.3	3.5
Current liabilities	3.7	4.1
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (net of tax)
Net loss (gain)	(1.9)	5.9	1.2
Amortization of unrecognized loss	(1.3)	(0.4)	(0.2)
Total recognized in other comprehensive income	(3.2)	5.5	1.0
Total recognized in net periodic benefit cost and other comprehensive income	$ 3.0	$ 9.8	$ 4.5