Income Taxes (Details) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
		Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate		21.00%			21.00%
Tax Cuts and Jobs Act of 2017, Net Operating Loss Deduction Limitation, Percent of Taxable income					80.00%
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit, Gross							$ 210,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Net Income Reduction							1,000,000
Unrecognized Tax Benefits [Abstract]
Unrecognized Tax Benefits - Permanent tax positions									$ 2,600,000	$ 2,000,000.0
Unrecognized tax benefit — Temporary tax positions									800,000	800,000
Total unrecognized tax benefit		$ 2,400,000	$ 2,800,000	$ 2,800,000	$ 2,400,000		2,400,000		3,400,000	2,800,000		$ 2,400,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1				2,800,000	2,400,000		5,300,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions				400,000	200,000		400,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				0	(100,000)		(300,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions				200,000	700,000		1,300,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions				0	(300,000)		(4,300,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				0	(100,000)		0
Balance at Dec. 31		$ 2,400,000	2,800,000	$ 3,400,000	$ 2,800,000		$ 2,400,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards									(2,200,000)	(2,100,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards									(1,200,000)	(1,100,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible									2,200,000
Amounts accrued for penalties related to unrecognized tax benefits									0	0		$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%	21.00%	[1]	35.00%	[1]
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent				6.20%	6.20%	[1]	5.10%	[1]
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	[2]			(5.20%)	(1.50%)	[1]	(1.80%)	[1]
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent				(3.60%)	0.00%	[1]	0.00%	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent				(1.80%)	(0.80%)	[1]	(1.00%)	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items				(0.70%)	(0.60%)	[1]	(2.30%)	[1]
Effective Income Tax Reconciliation, Adjustments Attributable to Tax Returns, Percent				0.70%	0.40%	[1]	0.80%	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent				0.00%	0.00%	[1]	0.00%	[1]
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent				0.00%	0.10%	[1]	0.00%	[1]
Effective Income Tax Rate Reconciliation, Percent				16.60%	24.80%	[1]	35.80%	[1]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)				$ 5,600,000	$ 7,600,000		$ 2,800,000
Current State and Local Tax Expense (Benefit)				4,700,000	1,700,000		0
Current Change In Unrecognized Tax Expense (Benefit)				500,000	200,000		(3,700,000)
Deferred Federal Income Tax Expense (Benefit)				3,000,000.0	15,600,000		32,900,000
Deferred State and Local Income Tax Expense (Benefit)				2,300,000	7,400,000		8,000,000.0
Deferred Change In Unrecognized Tax Expense (Benefit)				100,000	300,000		4,700,000
Deferred investment tax credits				(500,000)	(500,000)		(500,000)
Income Tax Expense (Benefit)				15,700,000	32,300,000		44,200,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items				17,400,000	24,000,000.0		(173,900,000)
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities				(11,900,000)	(700,000)		219,500,000
Deferred Other Tax Expense (Benefit)				(100,000)	0		0
Deferred Income Tax Expense (Benefit)				$ 5,400,000	$ 23,300,000		45,600,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment									304,100,000	281,100,000	[3]
Deferred Tax Liabilities, Regulatory Assets									54,900,000	55,400,000	[3]
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits									14,200,000	13,900,000	[3]
Deferred Tax Liabilities, Other									6,600,000	6,900,000	[3]
Deferred Tax Liabilities, Gross									379,800,000	357,300,000	[3]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Regulatory Liabilities									51,400,000	53,400,000	[3]
Deferred Tax Assets Rate Refund									8,800,000	1,300,000	[3]
Deferred Tax Assets Environmental Remediation									6,400,000	7,800,000	[3]
Deferred Tax Assets, Tax Credit Carryforwards									5,000,000.0	4,700,000	[3]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits									3,900,000	4,100,000	[3]
Deferred Tax Assets Deferred Investment Tax Credits									2,700,000	3,000,000.0	[3]
Deferred Tax Assets, Operating Loss Carryforwards									200,000	400,000	[3]
Deferred Tax Assets, Other									3,300,000	1,900,000	[3]
Deferred Tax Assets, Net of Valuation Allowance									81,700,000	76,600,000	[3]
Deferred Tax Liabilities, Net									298,100,000	280,700,000	[3]
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Tax Credit Carryforward, Amount									4,900,000	4,700,000
Potential Tax Adjustments			$ 0
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards									2,400,000	2,500,000
Tax Credit Carryforward Net Of Federal Detriment									$ 200,000	$ 0
Plant Related Regulatory Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit							149,000,000
Non-Plant Related Regulated Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit							41,000,000
Non-Plant Related Regulatory Asset [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Asset, Provisional Income Tax Expense (Benefit)							$ 23,000,000

[1] Prior periods have been reclassified to conform to current year presentation.

[2] Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.

[3] Prior periods have been reclassified to conform to current year presentation.