Pension and Other Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	$ 35.2	$ 56.1
Amortization of Prior Service (Cost) Credit	(0.3)	(0.3)
Amortization of Loss	(17.5)	(12.2)
Total Recognized in Other Comprehensive Income and Regulatory Assets	17.4	43.6
Postretirement Health and Life [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	(47.5)	6.9
Prior Service Credit Arising During the Period	(5.3)	0
Amortization of Prior Service (Cost) Credit	1.7	1.7
Amortization of Transition Obligation	(0.1)	(0.1)
Amortization of Loss	(7.5)	(8.5)
Total Recognized in Other Comprehensive Income and Regulatory Assets	$ (58.7)	$ 0