Quarterly report pursuant to Section 13 or 15(d)

BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details)

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BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 0.9 $ 1.0
Interest cost 6.7 7.1
Expected return on plan assets (9.6) (9.3)
Amortization of prior service cost 0.1 0.2
Recognized actuarial loss 5.0 3.8
Total net periodic benefit cost 3.1 2.8
Other Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0.1 0.1
Interest cost 0.2 0.2
Expected return on plan assets (0.3) (0.3)
Amortization of prior service cost (0.3) (0.3)
Recognized actuarial loss 0.4 0.4
Total net periodic benefit cost $ 0.1 $ 0.1