Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Qualified and nonqualified pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.1	$ 3.4	$ 6.1	$ 6.8	$ 13.1	$ 12.6
Interest cost	8.1	8.1	16.3	16.2	32.8	32.9
Expected return on plan assets	(12.1)	(11.7)	(24.3)	(23.3)	(48.7)	(45.6)
Prior service cost	0.0	0.0	0.0	0.0	0.1	0.3
Recognized net actuarial loss	3.7	5.7	7.5	11.4	18.0	18.9
Settlement costs	1.1	0.0	1.1	0.0	17.8	0.0
Net benefit cost	3.9	5.5	6.7	11.1	33.1	19.1
Estimated qualified pension plan cost for current fiscal year	8.1		8.1		8.1
Estimated nonqualified pension plan cost for current fiscal year	5.2		5.2		5.2
Postretirement benefits plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.2	0.3	0.5	0.6
Interest cost	0.9	0.9	1.8	1.8	3.5	3.6
Expected return on plan assets	(0.7)	(0.8)	(1.4)	(1.6)	(3.0)	(3.1)
Recognized net actuarial loss	0.3	0.3	0.5	0.6	1.1	0.7
Accretion of regulatory liability	0.2	0.2	0.4	0.4	1.0	0.8
Net benefit cost	0.8	$ 0.7	1.5	$ 1.5	3.1	$ 2.6
Estimated postretirement benefits cost for current fiscal year, before accretion of regulatory liability	$ 2.4		$ 2.4		$ 2.4