Pension and Post-retirement Plans: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Components of Net Periodic Benefit Cost
Service cost	$ 0.7	$ 1.4
Interest cost	2.1	2.9
Expected return on plan assets	(2.7)	(2.7)
Amortization of net (gain) loss	2.0	0.9
Amortization of prior service cost	(0.6)
Net periodic benefit cost	1.5	2.5
Estimated full year cash contributions to pension plan	3.5
Estimated contributions to be made	3.5
Post-retirement Benefits
Components of Net Periodic Benefit Cost
Service cost	0.3	0.3
Interest cost	0.5	0.8
Amortization of net (gain) loss	0.2	0.5
Net periodic benefit cost	$ 1.0	$ 1.6