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BENEFIT PLANS: (Details)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2019
USD ($)
plan
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Benefit plans      
Number of noncontributory defined benefit pension plans | plan 2    
Maximum      
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 0.8 $ 0.8  
Interest cost 1.3 1.2  
Expected return on plan assets (2.6) (2.7)  
Amortization of prior service cost (credit) 0.2 0.2  
Amortization of net loss (gain) 0.1 0.1  
Net periodic benefit cost (0.2) (0.4)  
Post-retirement Health care plans      
Defined benefit plan, net periodic benefit costs      
Interest cost 0.7 0.7  
Amortization of net loss (gain) (0.2) (0.2)  
Net periodic benefit cost 0.5 0.5  
Post-retirement Health care plans | Maximum      
Defined benefit plan, net periodic benefit costs      
Amortization of prior service cost (credit) $ 0.1 $ 0.1