BENEFIT PLANS: (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) plan	Mar. 31, 2019 USD ($)	Dec. 31, 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	0.4	$ 0.3
Interest cost	0.5	0.4
Expected return on plan assets	(0.7)	(0.8)
Net periodic benefit cost	0.2	(0.1)
Post-retirement Health care plans
Defined benefit plan, net periodic benefit costs
Interest cost	0.3	0.2
Net periodic benefit cost	0.3	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