Benefit Plans, Fair Values by Asset Category (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Weighted average asset allocations value [Abstract]
Fair value of plan assets	$ 320.2	$ 289.3
Equity Securities [Member]
Weighted average asset allocations value [Abstract]
Fair value of plan assets	179.2	174.1
Fixed Income Securities [Member]
Weighted average asset allocations value [Abstract]
Fair value of plan assets	116.0	94.7
Real Estate [Member]
Weighted average asset allocations value [Abstract]
Fair value of plan assets	2.4	1.9
Other [Member]
Weighted average asset allocations value [Abstract]
Fair value of plan assets	$ 22.6	$ 18.6