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BENEFIT PLANS - Components of Net Periodic Benefit Cost - Pension Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Weighted-average assumptions used to determine net periodic benefit cost for years ended December 31      
Expected return on plan assets 4.85% 4.85%  
Pension Plans, Defined Benefit      
Components of Net Periodic Pension Benefit Cost      
Service cost $ 2.3 $ 2.5 $ 4.0
Interest cost 32.9 34.0 21.1
Expected return on plan assets (28.1) (27.6) (30.1)
Amortization of prior service cost 1.0 1.4 1.4
Amortization of actuarial loss 5.1 5.6 4.2
Net periodic postretirement benefit cost 13.2 15.9 0.6
Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income      
Net actuarial loss (gain) (10.2) (9.4) 11.2
Reclassification of prior service cost (1.0) (1.4) (1.4)
Reclassification of actuarial loss (5.1) (5.6) (4.2)
Amount recognized in other comprehensive income (16.3) (16.4) 5.6
Amount recognized in net periodic pension benefit cost and other comprehensive income $ (3.1) $ (0.5) $ 6.2
Weighted-average assumptions used to determine net periodic benefit cost for years ended December 31      
Discount rate — PBO 5.00% 5.19% 2.92%
Discount rate — service cost 5.10% 5.29% 3.17%
Discount rate — interest cost 4.90% 5.09% 2.37%
Expected return on plan assets 4.85% 4.85% 4.00%
Weighted-average assumptions used to determine benefit obligation at December 31      
Discount rate 5.65% 5.00% 5.19%
Other Postretirement Benefit Plans, Defined Benefit      
Components of Net Periodic Pension Benefit Cost      
Service cost $ 2.2 $ 2.0 $ 2.2
Interest cost 2.2 2.0 0.9
Amortization of prior service cost 1.4 1.4 (0.3)
Amortization of actuarial loss (0.7) (1.5) (1.3)
Net periodic postretirement benefit cost 5.1 3.9 1.5
Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income      
Net actuarial loss (gain) (3.0) 3.6 (2.4)
Reclassification of prior service cost (1.4) (1.4) 0.3
Reclassification of actuarial loss 0.7 1.5 1.3
Amount recognized in other comprehensive income (3.7) 3.7 (0.8)
Amount recognized in net periodic pension benefit cost and other comprehensive income $ 1.4 $ 7.6 $ 0.7
Assumed Healthcare Cost Trend Rates at December 31      
Rate to which the cost trend rate gradually declines 4.50% 4.50% 4.50%
Year that the rate reaches the rate it is assumed to maintain 2035 2031 2030
Weighted-average assumptions used to determine net periodic benefit cost for years ended December 31      
Discount rate — PBO 4.90% 5.09% 2.59%
Discount rate — service cost 4.96% 5.15% 2.89%
Discount rate — interest cost 4.85% 5.03% 2.06%
Weighted-average assumptions used to determine benefit obligation at December 31      
Discount rate 5.51% 4.90% 5.08%
Other Postretirement Benefit Plans, Defined Benefit | Pre-65      
Assumed Healthcare Cost Trend Rates at December 31      
Healthcare cost trend rate assumed for next year (Pre-65/Post-65) 8.45% 10.20% 7.04%