Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) Plan	Dec. 31, 2018 USD ($) Plan	Dec. 31, 2017 USD ($) Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		$ 6.8	$ 6.9
Operating and maintenance expenses		285.3	282.7	$ 285.4
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent		(67.0)	(92.4)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		2.4	3.2
Accumulated Benefit Obligation at Dec. 31		481.1	445.8
Operating and maintenance expenses		0.6	0.7
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	$ 518.8	477.8	515.9
Service cost		8.8	9.7	9.8
Interest cost		20.1	18.4	19.7
Plan amendments		0.0	0.0
Actuarial loss		44.2	(34.8)
Benefit payments		(32.1)	(31.4)
Obligation at Dec. 31		518.8	477.8	515.9
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	458.2	392.2	433.2
Actual return on plan assets		80.2	(17.6)
Employer contributions		17.9	8.0
Benefit payments		(32.1)	(31.4)
Fair value of plan assets at Dec. 31		458.2	392.2	433.2
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		(60.6)	(85.6)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss		209.7	230.9
Prior service credit		(1.1)	(1.2)
Total		208.6	229.7
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets		11.0	12.9
Noncurrent regulatory assets		197.6	216.8
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Liabilities		0.0	0.0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Liabilities		0.0	0.0
Total		$ 208.6	$ 229.7
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		3.49%	4.31%
Expected average long-term increase in compensation level (as a percent)		3.75%	3.75%
Mortality table		Pri-2012	RP-2014
Cash Flows [Abstract]
Payment for Pension Benefits		$ 18.0	$ 8.0	24.0
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		8.8	9.7	9.8
Interest cost		20.1	18.4	19.7
Expected return on plan assets		(28.6)	(28.3)	(27.9)
Amortization of prior service cost		(0.1)	(0.1)	0.0
Amortization of net loss		11.3	14.1	13.0
Net periodic pension cost		13.9	17.0	14.6
Net benefit cost recognized for financial reporting		$ 14.8	$ 14.8	$ 14.9
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)		4.31%	3.63%	4.13%
Expected average long-term increase in compensation level (as a percent)		3.75%	3.75%	3.75%
Expected average long-term rate of return on assets (as a percent)		6.78%	6.78%	6.78%
Plan participant's contributions		$ 0.0	$ 0.0
Plan participant's contributions		0.0	0.0
Liability, Defined Benefit Plan, Noncurrent		(60.6)	(85.6)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position		(60.6)	(85.6)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		2.4	3.2	$ 0.0
Defined Benefit Plan, Costs Not Recognized Due To Regulation		$ 0.9	$ (2.2)	$ 0.3
Subsequent Event | Pension Plan [Member]
Cash Flows [Abstract]
Payment for Pension Benefits	14.0
Parent Company [Member] | Pension Plan [Member]
Cash Flows [Abstract]
Number of pension plans to which contributions were made | Plan		4	4	4
Payment for Pension Benefits		$ 154.0	$ 150.0	$ 162.0
Parent Company [Member] | Subsequent Event | Pension Plan [Member]
Cash Flows [Abstract]
Payment for Pension Benefits	$ 150.0