PENSION AND POST-RETIREMENT PLANS - STATUS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in Benefit Obligation:
Benefit obligation at beginning of year	$ 225.4	$ 220.2
Service cost	4.0	3.9	$ 3.3
Interest cost	9.7	9.7	9.5
Settlements	(0.3)
Actuarial loss (gain)	13.9	4.2
Benefits paid, net of subsidies received	(18.9)	(11.1)
Expenses paid	(1.7)	(1.5)
Benefit obligation at end of year	232.1	225.4	220.2
Changes in plan assets:
Balance at the beginning of the period	178.8	168.9
Actual return on plan assets	25.8	15.0
Plan settlements	(0.3)
Employer contribution	3.0	7.5	4.7
Benefits paid, net of subsidies received	(18.9)	(11.1)
Expenses paid	(1.7)	(1.5)
Balance at the end of the period	186.7	178.8	168.9
Funded Status and Recognized Liability	(45.4)	(46.6)
Other Post-retirement Benefits
Change in Benefit Obligation:
Benefit obligation at beginning of year	25.2	60.5
Service cost	0.5	1.5	1.5
Interest cost	1.1	2.7	2.5
Plan Amendments		(38.4)
Plan participants' contributions	1.1	1.0
Actuarial loss (gain)	1.9	1.9
Benefits paid, net of subsidies received	(2.1)	(4.0)
Benefit obligation at end of year	27.7	25.2	$ 60.5
Changes in plan assets:
Plan participants' contributions	1.1	1.0
Employer contribution	1.0	3.0
Benefits paid, net of subsidies received	(2.1)	(4.0)
Funded Status and Recognized Liability	(27.7)	(25.2)
Qualified Plan | Pension Benefits
Change in Benefit Obligation:
Benefit obligation at beginning of year	223.2
Benefit obligation at end of year	229.9	223.2
Changes in plan assets:
Balance at the beginning of the period	175.9
Balance at the end of the period	$ 184.7	$ 175.9