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BENEFIT PLANS: (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2018
USD ($)
plan
Mar. 31, 2017
USD ($)
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 $ 0.3 $ 0.2
Interest cost 0.4 0.3
Expected return on plan assets (0.9) (0.7)
Net periodic benefit cost (0.2) (0.2)
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 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