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BENEFIT PLANS: (Details)
$ in Millions
 6 Months Ended
Jun. 30, 2018
USD ($)
plan
Jun. 30, 2017
USD ($)
Benefit plans    
Number of noncontributory defined benefit pension plans | plan 2  
Post retirement defined benefit plans    
Defined benefit plan, net periodic benefit costs    
Service cost $ 0.5 $ 0.4
Interest cost 0.8 0.7
Expected return on plan assets (1.8) (1.4)
Amortization of prior service cost (credit) 0.1 0.1
Amortization of net loss (gain) 0.1 0.1
Net periodic benefit cost (0.3) (0.1)
Post retirement defined benefit plans | Maximum    
Defined benefit plan, net periodic benefit costs    
Amortization of prior service cost (credit) 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 0.5 0.5
Amortization of net loss (gain) (0.1) (0.1)
Net periodic benefit cost 0.4 0.4
Post-retirement health care plans | Maximum    
Defined benefit plan, net periodic benefit costs    
Amortization of prior service cost (credit) $ 0.1 $ 0.1