Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Plans [Member]
Change in Benefit Obligation
Benefit Obligation at January 1	$ 1,222.8	$ 1,160.7
Service cost	15.9	23.7	23.3
Interest cost	67.6	68.4	72.3
Participants' contributions	0	0
Plan amendments	0	0
Actuarial loss (gain)	98.0	53.4
Curtailments	0	0
Gross benefits paid	(73.7)	(83.4)
Benefit Obligation at December 31	1,330.6	1,222.8	1,160.7
OPEB [Member]
Change in Benefit Obligation
Benefit Obligation at January 1	368.3	374.7
Service cost	10.4	11.2	8.7
Interest cost	20.8	21.2	20.5
Participants' contributions	11.6	6.5
Plan amendments	0.4	0.3
Actuarial loss (gain)	7.6	(23.8)
Curtailments	0	(1.0)
Gross benefits paid	(30.3)	(21.8)
Federal subsidy on benefits paid	0.9	1.0
Benefit Obligation at December 31	$ 389.7	$ 368.3	$ 374.7