Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Qualified and nonqualified pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.4	$ 3.3	$ 13.3	$ 10.4
Interest cost	7.9	7.7	31.3	29.1
Expected return on plan assets	(9.5)	(7.7)	(33.6)	(27.4)
Prior-service and other costs	0.3	0.3	1.1	1.9
Recognized net actuarial loss	6.8	5.4	26.3	14.9
Pension expene / Postretirement benefits expense	8.9	9.0	38.4	28.9
Estimated qualified pension plan expense for current fiscal year	32.2		32.2
Estimated nonqualified pension plan expense for current fiscal year	3.4		3.4
Postretirement benefits plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.2	0.1	0.8	0.4
Interest cost	1.0	0.9	3.9	4.0
Expected return on plan assets	(0.7)	(0.5)	(2.5)	(2.4)
Amortization of transition obligation	0	0.4	1.2	1.8
Amortization of losses	0.7	0.6	2.6	0.8
Accretion of regulatory liability	0.2	0.2	0.8	0.8
Pension expene / Postretirement benefits expense	1.4	1.7	6.8	5.4
Estimated postretirement benefits expense for current fiscal year, before accretion of regulatory liability	5.0		5.0
Estimated accretion of regulatory liability for current fiscal year	$ 0.8		$ 0.8