Employee Benefit Plans OPEB Changes in Assets Measured at Fair Value Using Signficant Unobservable Inputs (Details) - Other Postretirement Benefit Plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Plan Participant Contributions	$ 0.2	$ 4.0
Defined Benefit Plan, Plan Assets, Benefits Paid	10.3	15.6
Fair Value, Inputs, Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance of Assets Measured with Unobservable Inputs	10.8	11.2
Actual Return on Plan Assets Still Held	0.1	0.1
Plan Participant Contributions	9.9	15.1
Defined Benefit Plan, Plan Assets, Benefits Paid	10.3	15.6
Ending Balance of Assets Measured with Unobservable Inputs	$ 10.5	$ 10.8