Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Qualified and nonqualified pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.3	$ 2.2	$ 10.4	$ 9.1
Interest cost	7.7	7.1	29.1	27.9
Expected return on plan assets	(7.7)	(6.3)	(27.4)	(24.3)
Prior service and other costs	0.2	0.3	1.0	1.1
Recognized net actuarial loss	5.4	2.6	14.9	9.0
Curtailment costs	0.1	0	0.9	2.2
Pension expene / Postretirement benefits expense	9.0	5.9	28.9	25.0
Estimated qualified pension plan expense for current fiscal year	34.1		34.1
Estimated nonqualified pension plan expense for current fiscal year	2.0		2.0
Postretirement benefits plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.2	0.4	0.6
Interest cost	0.9	1.1	4.0	4.3
Expected return on plan assets	(0.5)	(0.7)	(2.4)	(2.6)
Amortization of transition obligation	0.4	0.5	1.8	1.9
Amortization of (gains) losses	0.6	0.1	0.8	0.5
Curtailment costs	0	0	0	0.3
Accretion of regulatory liability	0.2	0.2	0.8	0.8
Pension expene / Postretirement benefits expense	1.7	1.4	5.4	5.8
Estimated postretirement benefits expense for current fiscal year, before accretion of regulatory liability	6.0		6.0
Estimated accretion of regulatory liability for current fiscal year	$ 0.8		$ 0.8