Employee Benefits (Details) - Other Postretirement Benefit Plans, Defined Benefit [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.1	$ 1.3	$ 0.2	$ 2.6
Interest cost	10.9	12.8	21.8	25.6
Expected return on plan assets	(15.1)	(14.8)	(30.2)	(29.6)
Amortization of prior service credit	(0.1)	0.0	(0.2)	0.0
Amortization of net pension loss	3.7	3.9	7.4	7.8
Total periodic pension cost	(0.5)	$ 3.2	(1.0)	$ 6.4
Expected future benefit payments, remainder of the year	$ 15.4		15.4
Contributions by employer			$ 2.3