Employee Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Cost	$ 11.5	$ 11.1	$ 11.0
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Required amount of funding of periodic pension cost	3.0
Contribution to defined benefit pension plan	4.7	3.0
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	3.0
Minimum amount of recognized expense each year associated with retirement plan	$ 3.0
Future rate recovery	in excess of $6.0 million
Precluded future recovery of annual pension costs, lower range	$ 3.0
Precluded future recovery of annual pension costs, upper range	$ 6.0
Recognition of regulatory assets (in excess of)	in excess of $6.0 million
Reduction of regulatory assets (less than)	less than $3.0 million
Pension plan costs charged to expense, lower range	$ 3.0
Pension plan costs charged to expense, upper range	6.0
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution to defined benefit pension plan	0.1	0.2
SRP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution to defined benefit pension plan	$ 0.0	$ 0.8