Pension Plans and Other Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit costs
Service cost	$ 8.8	$ 7.8	$ 17.7	$ 15.6
Interest cost	12.3	12.6	24.5	25.1
Expected return on plan assets	(10.7)	(9.7)	(21.4)	(19.3)
Prior service cost	1.1	1.2	2.2	2.3
Recognized net actuarial (gain) loss	11.2	9.5	22.3	19.2
Settlement charges	0	0.2	0	0.2
Net period benefit costs before regulatory adjustment	22.7	21.6	45.3	43.1
Regulatory adjustment	(3.8)	(5.9)	(7.7)	(12.3)
Net periodic benefit costs	18.9	15.7	37.6	30.8
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit costs
Service cost	0.8	0.8	1.6	1.6
Interest cost	2.0	1.9	3.9	3.9
Expected return on plan assets	(0.4)	(0.5)	(0.9)	(0.9)
Prior service cost	1.8	1.8	3.6	3.6
Recognized net actuarial (gain) loss	0	(0.2)	0	(0.3)
Transition obligation	0.2	0.4	0.5	0.7
Net period benefit costs before regulatory adjustment	4.4	4.2	8.7	8.6
Regulatory adjustment	0.3	0.1	0.7	0.3
Net periodic benefit costs	$ 4.7	$ 4.3	$ 9.4	$ 8.9