Employee Benefit Plans OPEB Plan Assets, Fair Value Measurements by Input Level (Details) - Other Postretirement Benefit Plans - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans
Life Settlement Contracts, Fair Value Method, Face Value	$ 10.5	$ 10.8
Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans
Life Settlement Contracts, Fair Value Method, Face Value	0.0	0.0
Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans
Life Settlement Contracts, Fair Value Method, Face Value	0.0	0.0
Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans
Life Settlement Contracts, Fair Value Method, Face Value	$ 10.5	$ 10.8