Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Qualified and nonqualified pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.7	$ 2.9	$ 10.5	$ 8.9	$ 13.4	$ 12.4
Interest cost	8.3	8.3	24.5	25.0	32.9	32.6
Expected return on plan assets	(12.3)	(11.4)	(35.6)	(32.7)	(46.5)	(42.2)
Prior service cost	0.1	0.1	0.1	0.4	0.3	0.7
Recognized net actuarial loss	6.3	3.7	17.7	11.6	21.5	18.8
Net benefit cost	6.1	3.6	17.2	13.2	21.6	22.3
Estimated qualified pension plan cost for current fiscal year	19.0		19.0		19.0
Estimated nonqualified pension plan cost for current fiscal year	3.9		3.9		3.9
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year			69.0
Postretirement benefits plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.2	0.4	0.5	0.5	0.6
Interest cost	0.8	0.9	2.6	2.9	3.5	3.8
Expected return on plan assets	(0.7)	(0.8)	(2.3)	(2.3)	(3.0)	(3.0)
Recognized net actuarial loss (gain)	0.3	(0.1)	0.9	0.5	1.1	1.3
Accretion of regulatory liability	0.3	0.2	0.7	0.6	0.9	0.6
Net benefit cost	0.8	$ 0.4	2.3	$ 2.2	3.0	$ 3.3
Estimated postretirement benefits cost for current fiscal year, before accretion of regulatory liability	$ 2.1		$ 2.1		$ 2.1