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BENEFIT PLANS: (Details)
$ in Millions
 9 Months Ended
Sep. 30, 2021
USD ($)
plan
Sep. 30, 2020
USD ($)
Benefit plans    
Number of noncontributory defined benefit pension plans | plan 2  
Post retirement defined benefit plans and defined contribution plan    
Defined benefit plan, net periodic benefit costs    
Service cost $ 1.1 $ 1.0
Interest cost 1.2 1.2
Expected return on plan assets (2.6) (2.1)
Amortization of prior service cost / (credit) 0.1 0.1
Amortization of net loss/(gain) 0.2 0.2
Net periodic benefit cost 0.0 0.4
Post-retirement Health care plans    
Defined benefit plan, net periodic benefit costs    
Interest cost 1.3 0.9
Amortization of net loss/(gain) 0.1  
Net periodic benefit cost $ 1.4 $ 0.9