Employee Benefits, Defined Benefit Plans Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs	$ 12.0	$ 20.0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring [Member]
VEBA Postretirement Medical Obligations
Fair value of deferred compensation assets	9.7	$ 8.2
Voluntary Employees' beneficiary Association (VEBA) | Union VEBA
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs	17.1
Maximum contribution threshold	12.8
Accrued Veba contingent contribution - total	12.0
Voluntary Employees' beneficiary Association (VEBA) | Salaried VEBA
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs	2.9
Maximum contribution threshold	$ 2.9