Pension and Post-retirement Plans: (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 2.0	$ 4.1
Interest cost	6.8	8.1
Expected return on plan assets	(8.0)	(8.4)
Amortization of net (gain) loss	6.0	2.9
Amortization of prior service cost	(1.3)
Net periodic benefit cost	5.5	6.7
Cash contributed to pension plan	13.3
Estimated full year cash contributions to pension plan	13.3
Post-retirement Benefits
Components of net periodic benefit cost
Service cost	0.7	0.7
Interest cost	1.7	2.2
Amortization of net (gain) loss	1.0	1.5
Amortization of prior service cost		0.1
Net periodic benefit cost	$ 3.4	$ 4.5