Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 3.0
Defined Contribution Plan, Cost	2.7	$ 2.7	$ 8.0	$ 8.1
Retirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.7	0.8	2.5	2.6
Interest cost	1.2	1.2	3.4	3.5
Expected return on plan assets	(1.7)	(2.0)	(5.7)	(5.9)
Amortization of unrecognized net loss	0.3	0.4	1.1	1.3
Settlement charge	0.4	0.4	2.6	1.5
Net periodic benefit cost	0.9	0.8	3.9	3.0
PBOP
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.0	0.0	0.1	0.1
Interest cost	0.4	0.4	1.1	1.2
Expected return on plan assets	(1.2)	(1.1)	(3.5)	(3.3)
Amortization of unrecognized net loss	0.0	0.0	0.0	0.0
Settlement charge	0.0	0.0	0.0	0.0
Net periodic benefit cost	$ (0.8)	$ (0.7)	$ (2.3)	$ (2.0)