Pension Plans and Other Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 8.9	$ 7.8	$ 26.6	$ 23.4
Interest cost	12.2	12.4	36.7	37.5
Expected return on plan assets	(10.7)	(9.5)	(32.1)	(28.8)
Prior service cost	1.1	1.1	3.3	3.4
Recognized net actuarial (gain) loss	11.1	9.7	33.4	28.9
Transition obligation	0	0	0	0
Settlement charge	0	10.0	0	10.2
Net period benefit costs before regulatory adjustment	22.6	31.5	67.9	74.6
Regulatory adjustment	(3.9)	(12.8)	(11.6)	(25.1)
Net periodic benefit costs	18.7	18.7	56.3	49.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.8	0.7	2.4	2.3
Interest cost	1.9	2.0	5.8	5.9
Expected return on plan assets	(0.4)	(0.4)	(1.3)	(1.3)
Prior service cost	1.8	1.8	5.4	5.4
Recognized net actuarial (gain) loss	0	(0.1)	0	(0.4)
Transition obligation	0.3	0.3	0.8	1.0
Settlement charge	0	0	0	0
Net period benefit costs before regulatory adjustment	4.4	4.3	13.1	12.9
Regulatory adjustment	0.3	0.4	1.0	0.7
Net periodic benefit costs	$ 4.7	$ 4.7	$ 14.1	$ 13.6