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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 0.9 $ 1.0 $ 1.8 $ 2.0
Interest cost 6.7 7.1 13.4 14.2
Expected return on plan assets (9.6) (9.3) (19.2) (18.6)
Amortization of prior service cost 0.1 0.2 0.2 0.4
Recognized actuarial loss 5.0 3.8 10.0 7.6
Total net periodic benefit cost 3.1 2.8 6.2 5.6
Other Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.1 0.1 0.2 0.2
Interest cost 0.2 0.2 0.4 0.4
Expected return on plan assets (0.3) (0.3) (0.6) (0.6)
Amortization of prior service cost (0.3) (0.3) (0.6) (0.6)
Recognized actuarial loss 0.4 0.4 0.8 0.8
Total net periodic benefit cost $ 0.1 $ 0.1 $ 0.2 $ 0.2