BENEFIT PLANS: (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) plan	Jun. 30, 2019 USD ($)
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	0.7	0.5
Interest cost	0.9	0.9
Expected return on plan assets	(1.4)	(1.7)
Amortization of prior service cost (credit)	0.1	0.1
Amortization of net loss (gain)	0.1	0.1
Net periodic benefit cost	0.4	(0.1)
Post-retirement Health care plans
Defined benefit plan, net periodic benefit costs
Interest cost	0.6	0.5
Amortization of net loss (gain)		(0.1)
Net periodic benefit cost	0.6	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