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BENEFIT PLANS: (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2019
USD ($)
plan
Mar. 31, 2018
USD ($)
Benefit plans    
Number of noncontributory defined benefit pension plans | plan 2  
Defined benefit plan, net periodic benefit costs    
Amortization of net loss (gain) $ 0.1 $ 0.1
Post retirement defined benefit plans and defined contribution plan    
Defined benefit plan, net periodic benefit costs    
Service cost 0.3 0.3
Interest cost 0.4 0.4
Expected return on plan assets (0.8) (0.9)
Net periodic benefit cost (0.1) (0.2)
Post retirement defined benefit plans and defined contribution plan | Maximum    
Defined benefit plan, net periodic benefit costs    
Amortization of net actuarial loss 0.1 0.1
Amortization of net loss (gain) 0.1 0.1
Post-retirement Health care plans    
Defined benefit plan, net periodic benefit costs    
Interest cost 0.2 0.2
Net periodic benefit cost 0.2 0.2
Post-retirement Health care plans | Maximum    
Defined benefit plan, net periodic benefit costs    
Amortization of prior service cost (credit) $ 0.1 $ 0.1