Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Contribution Plan, Cost	$ 3.0	$ 2.8	$ 8.8	$ 8.4
Pension and SRP Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.7	0.7	2.1	2.3
Interest cost	0.7	0.9	2.2	3.1
Expected return on plan assets	(1.4)	(1.6)	(4.7)	(4.8)
Amortization of unrecognized net loss	0.8	0.4	1.6	1.6
Settlement charge	0.6	0.6	2.2	2.7
Net periodic benefit cost	1.4	1.0	3.4	4.9
PBOP
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.3	0.4	0.9	1.1
Expected return on plan assets	(0.8)	(0.6)	(2.4)	(2.2)
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	(0.5)	$ (0.2)	(1.5)	$ (1.1)
Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Plan Assets, Contributions by Employer			3.6
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	$ 0.0		$ 0.0