Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Document Period End Date			Jun. 30, 2017
Components of net periodic benefit cost
Contributions to qualified and non qualified pension plans			$ 13.6	$ 1.7
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 3.6	$ 0.9	7.2	1.8
Interest cost	7.9	6.2	15.8	12.4
Expected return on plan assets	(8.6)	(6.8)	(17.2)	(13.5)
Recognized net actuarial loss	1.4	1.4	2.8	2.8
Net periodic pension benefit cost	4.3	1.7	8.6	3.5
Other Postretirement Benefits Plan [Member]
Components of net periodic benefit cost
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.1	0.1	0.2	0.2
Expected return on plan assets	0.0	0.0	0.0	0.0
Recognized net actuarial loss	0.0	0.0	0.0	0.1
Net periodic pension benefit cost	$ 0.1	$ 0.1	$ 0.2	$ 0.3