Pension and Post-retirement Plans: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2013 Pension Benefits	Sep. 30, 2012 Pension Benefits	Sep. 30, 2013 Post-retirement Benefits	Sep. 30, 2012 Post-retirement Benefits
Components of Net Periodic Benefit Cost
Service cost			$ 2.1	$ 2.0	$ 0.8	$ 0.7
Interest cost			6.5	6.8	1.6	1.7
Expected return on plan assets			(8.9)	(8.0)
Amortization of net loss	(1.3)	(3.5)	5.1	6.0	0.2	1.0
Amortization of prior service cost			(1.7)	(1.3)
Net periodic benefit cost			3.1	5.5	2.6	3.4
Estimated full year cash contributions to pension plan			3.5
Employer contribution			$ 3.5