Benefit Plans, Amounts Recognized in Accumulated Other Comprehensive Income, net of Related Tax Effects (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized in accumulated other comprehensive income [Abstract]
Amount recognized end of year	$ 86.5	$ 78.0
Accumulated benefit obligation	435.4	394.5
Pension Benefits [Member]
Amounts recognized in accumulated other comprehensive income [Abstract]
Net actuarial (gain) loss	91.4	82.1
Prior service cost	(0.1)	(0.1)
Amount recognized end of year	91.3	82.0
Post-retirement Benefits [Member]
Amounts recognized in accumulated other comprehensive income [Abstract]
Net actuarial (gain) loss	(4.1)	(1.7)
Prior service cost	(0.7)	(2.4)
Amount recognized end of year	$ (4.8)	$ (4.1)