Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Change in Benefit Obligation
Benefit Obligation at December 31	$ 1,507.1	$ 1,329.4
Pension Plans [Member]
Change in Benefit Obligation
Benefit Obligation at January 1	1,330.6	1,222.8
Service cost	21.7	15.9	23.7
Interest cost	65.5	67.6	68.4
Participants' contributions	0	0
Plan amendments	0	0
Actuarial loss (gain)	166.5	98.0
Curtailments	31.4	0
Gross benefits paid	(107.2)	(73.7)
Benefit Obligation at December 31	1,508.5	1,330.6	1,222.8
OPEB [Member]
Change in Benefit Obligation
Benefit Obligation at January 1	389.7	368.3
Service cost	10.3	10.4	11.2
Interest cost	20.3	20.8	21.2
Participants' contributions	9.6	11.6
Plan amendments	0.5	0.4
Actuarial loss (gain)	(23.8)	7.6
Curtailments	0	0
Gross benefits paid	(26.3)	(30.3)
Federal subsidy on benefits paid	0.9	0.9
Benefit Obligation at December 31	$ 381.2	$ 389.7	$ 368.3