Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Plan	Dec. 31, 2017 USD ($) Plan	Dec. 31, 2016 USD ($) Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement			$ 6,900,000	$ 0
Operating and maintenance expenses			282,700,000	285,400,000	$ 265,500,000
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent			(92,400,000)	(90,300,000)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement			3,300,000
Accumulated Benefit Obligation at Dec. 31			445,800,000	478,800,000
Operating and maintenance expenses			0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	$ 477,800,000	$ 477,800,000	515,900,000	483,600,000
Service cost			9,700,000	9,800,000	9,800,000
Interest cost			18,400,000	19,700,000	21,200,000
Plan amendments			0	(1,000,000)
Actuarial loss			(34,800,000)	31,200,000
Benefit payments			(31,400,000)	(27,400,000)
Obligation at Dec. 31			477,800,000	515,900,000	483,600,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	392,200,000	392,200,000	433,200,000	380,400,000
Actual return on plan assets			(17,600,000)	56,700,000
Employer contributions			8,000,000	23,500,000
Benefit payments			(31,400,000)	(27,400,000)
Fair value of plan assets at Dec. 31			392,200,000	433,200,000	380,400,000
Funded Status of Plans at Dec. 31 [Abstract]
Funded status			(85,600,000)	(82,700,000)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss			230,900,000	237,000,000
Prior service credit			(1,200,000)	(1,300,000)
Total			229,700,000	235,700,000
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			12,900,000	13,900,000
Noncurrent regulatory assets			216,800,000	221,800,000
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Liabilities			0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Liabilities			0	0
Total			$ 229,700,000	$ 235,700,000
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)			4.31%	3.63%
Expected average long-term increase in compensation level (as a percent)			3.75%	3.75%
Mortality table			RP-2014	RP-2014
Cash Flows [Abstract]
Payment for Pension Benefits			$ 8,000,000	$ 24,000,000	18,000,000
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			9,700,000	9,800,000	9,800,000
Interest cost			18,400,000	19,700,000	21,200,000
Expected return on plan assets			(28,300,000)	(27,900,000)	(27,600,000)
Amortization of prior service cost			(100,000)	0	0
Amortization of net loss			14,100,000	13,000,000	12,000,000
Net periodic pension cost			17,000,000	14,600,000	15,400,000
Net benefit cost recognized for financial reporting			$ 14,800,000	$ 14,900,000	$ 17,400,000
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)			3.63%	4.13%	4.66%
Expected average long-term increase in compensation level (as a percent)			3.75%	3.75%	4.00%
Expected average long-term rate of return on assets (as a percent)			6.78%	6.78%	6.78%
Plan participant's contributions			$ 0	$ 0
Plan participant's contributions			0	0
Liability, Defined Benefit Plan, Noncurrent			(85,600,000)	(82,700,000)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position			(85,600,000)	(82,700,000)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement			3,200,000	0	$ 0
Defined Benefit Plan, Costs Not Recognized Due To Regulation			$ (2,200,000)	$ 300,000	$ 2,000,000
Pension Plan [Member] | Subsequent Event
Cash Flows [Abstract]
Payment for Pension Benefits	$ 17,000,000
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		$ 150,000,000	$ 150,000,000	$ 162,000,000	$ 125,000,000