BENEFIT PLANS: (Details) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) plan	Sep. 30, 2015 USD ($)
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	$ 0.6	$ 0.7
Interest cost	0.7	0.7
Expected return on plan assets	(1.8)	(2.4)
Amortization of net loss (gain)	0.2	(0.1)
Amortization of prior service cost (credit)	0.1	0.1
Net periodic benefit cost	(0.2)	(1.0)
Post-retirement Health care plans
Defined benefit plan, net periodic benefit costs
Interest cost	0.4	1.0
Amortization of net loss (gain)	(0.3)	(0.2)
Net periodic benefit cost	0.1	0.8
Post-retirement Health care plans | Maximum
Defined benefit plan, net periodic benefit costs
Amortization of prior service cost (credit)	$ 0.1	$ 0.1