Employee Benefits (Details) - Other Postretirement Benefit Plans, Defined Benefit [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.3	$ 1.6	$ 3.9	$ 4.8
Interest cost	12.8	14.0	38.4	42.0
Expected return on plan assets	(14.8)	(14.2)	(44.4)	(42.4)
Amortization of net pension loss	3.9	3.5	11.7	10.3
Total periodic pension cost	3.2	$ 4.9	9.6	$ 14.7
Expected future benefit payments, remainder of the year	$ 54.1		54.1
Contributions by employer			$ 39.9