Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Qualified and nonqualified pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.0	$ 3.6	$ 6.0	$ 7.1	$ 12.8	$ 13.5
Interest cost	8.3	7.7	16.7	15.5	31.8	31.4
Expected return on plan assets	(10.7)	(9.6)	(21.3)	(19.1)	(40.3)	(35.2)
Prior service and other costs	0.2	0.3	0.3	0.6	0.8	1.0
Recognized net actuarial loss	4.0	6.9	7.9	13.7	23.1	27.8
Net benefit cost	4.8	8.9	9.6	17.8	28.2	38.5
Estimated qualified pension plan cost for current fiscal year	13.6		13.6		13.6
Projected current year qualified pension plan funding			43.0
Estimated nonqualified pension plan cost for current fiscal year	4.0		4.0		4.0
Postretirement benefits plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.2	0.3	0.4	0.6	0.8
Interest cost	1.0	0.9	2.0	1.9	3.8	3.9
Expected return on plan assets	(0.7)	(0.6)	(1.5)	(1.3)	(2.8)	(2.5)
Amortization of transition obligation	0	0	0	0	0	0.8
Recognized net actuarial loss	0.3	0.7	0.6	1.4	2.2	2.8
Accretion of regulatory liability	0.2	0.1	0.4	0.3	0.6	0.2
Net benefit cost	0.9	1.3	1.8	2.7	4.4	6.0
Estimated postretirement benefits cost for current fiscal year, before accretion of regulatory liability	$ 2.1		$ 2.1		$ 2.1