EMPLOYEE BENEFITS - AMOUNTS RECOGNIZED ON THE BALANCE SHEETS (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension and OPEB assets	$ 62.0	$ 114.8
Pension and OPEB obligations	24.0	25.4
Pension Costs
Defined Benefit Plan Disclosure [Line Items]
Pension and OPEB assets	15.6	89.0
Pension and OPEB obligations	7.9	7.6
Total net assets	7.7	81.4
OPEB Costs
Defined Benefit Plan Disclosure [Line Items]
Pension and OPEB assets	46.4	25.8
Pension and OPEB obligations	16.1	17.8
Total net assets	$ 30.3	$ 8.0