Employee Benefits (Details) (USD $) In Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Employee Benefits [Abstract]
Estimated qualified pension plan expense for current fiscal year	$ 22.8		$ 22.8		$ 22.8
Estimated nonqualified pension plan expense for current fiscal year	0.8		0.8		0.8
Estimated postretirement benefits expense for current fiscal year, before accretion of regulatory liability	4.8		4.8		4.8
Estimated accretion of regulatory liability for current fiscal year	0.8		0.8		0.8
Qualified and nonqualified pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.2	2.5	4.4	4.9	8.8	9.8
Interest cost	7.1	7.9	14.2	15.8	27.1	31.1
Expected return on plan assets	(6.3)	(6.4)	(12.6)	(12.8)	(24.2)	(25.4)
Prior service and other costs	0.3	0.3	0.6	0.6	1.1	1.2
Recognized net-actuarial loss	2.6	1.7	5.2	3.5	9.9	7.2
Curtailment costs	0	0.1	0	0.3	2.1	(0.3)
Special-termination benefits	0	0	0	0	0	2.0
Pension expense/Postretirement benefits expense	5.9	6.1	11.8	12.3	24.8	25.6
Postretirement benefits plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.3	0.3	0.6	0.6
Interest cost	1.1	1.2	2.2	2.4	4.2	4.6
Expected return on plan assets	(0.6)	(0.6)	(1.3)	(1.1)	(2.6)	(2.2)
Amortization of transition obligation	0.4	0.4	0.9	0.9	1.9	1.9
Amortization of losses	0.2	0.2	0.3	0.4	0.5	0.8
Curtailment costs	0	0	0	0	0.3	0
Accretion of regulatory liability	0.2	0.2	0.4	0.4	0.8	0.8
Pension expense/Postretirement benefits expense	$ 1.4	$ 1.5	$ 2.8	$ 3.3	$ 5.7	$ 6.5