Pension and Other Postemployment Benefit (OPEB) Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits [Member]
Components of net periodic expense of Pension and OPEB plans
Service cost	$ 0.6	$ 0.6
Interest cost	1.7	1.8
Expected return on plan assets	(2.2)	(2.1)
Amortization of net loss (gain)	0.7	0.8
Amortization of prior service (credit)	0	0
Net periodic expense (benefit)	0.8	1.1
Other Benefits [Member]
Components of net periodic expense of Pension and OPEB plans
Service cost	0	0
Interest cost	0	0
Expected return on plan assets	0	0
Amortization of net loss (gain)	(3.2)	(0.7)
Amortization of prior service (credit)	(0.5)	(1.8)
Net periodic expense (benefit)	$ (3.7)	$ (2.5)