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, 2016	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate		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		$ 559,000,000
Tax Cuts and Jobs Act, Incomplete Accounting, Provisional Income Tax Expense (Benefit)		8,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Net Income Reduction		2,000,000
Tax Audits [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued										$ 700,000	$ 700,000		$ 500,000	$ 900,000
Interest Income (Expense) related to unrecognized tax benefits				$ 0	$ (200,000)		$ (1,400,000)
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions										3,700,000	3,000,000.0
Unrecognized tax benefit — Temporary tax positions										1,500,000	1,500,000
Total unrecognized tax benefit		4,300,000	$ 4,500,000	5,200,000	4,300,000		4,300,000		$ 28,700,000	5,200,000	4,500,000		4,300,000	$ 28,700,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1				4,500,000	4,300,000		28,700,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions				700,000	600,000		900,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				(100,000)	(100,000)		(600,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions				200,000	100,000		1,300,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions				(100,000)	(300,000)		(19,900,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				0	(100,000)		(6,100,000)
Balance at Dec. 31		4,300,000	4,500,000	$ 5,200,000	$ 4,500,000		$ 4,300,000		$ 28,700,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards										(4,400,000)	(3,800,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards										(1,400,000)	(800,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible										3,700,000
Unrecognized Tax Benefits, Income Tax Penalties Accrued										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]	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent				2.20%	2.30%	[1]	2.00%	[1]
Effective Income Tax Rate Reconciliation, Tax Credit, Percent				(7.90%)	0.00%	[1]	0.00%	[1]
Effective Income Tax Reconciliation, Adjustments Attributable to Tax Returns, Percent				0.10%	1.50%	[1]	0.40%	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent				(0.60%)	(0.70%)	[1]	(0.60%)	[1]
Amortization of excess nonplant deferred taxes				(0.90%)	(1.20%)	[1]	0.00%	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	[2]			(5.00%)	(4.80%)	[1]	(0.90%)	[1]
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent				0.00%	0.00%	[1]	(3.50%)	[1]
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent				0.00%	0.20%	[1]	(0.50%)	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits				0.20%	0.10%	[1]	(1.00%)	[1]
Effective Income Tax Rate Reconciliation, Percent				8.90%	15.40%	[1]	30.10%	[1]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)				$ (3,900,000)	$ 12,300,000		$ (20,900,000)
Current State and Local Tax Expense (Benefit)				600,000	2,300,000		(12,800,000)
Current Change In Unrecognized Tax Expense (Benefit)				0	2,300,000		(24,300,000)
Deferred Federal Income Tax Expense (Benefit)				22,300,000	20,500,000		89,900,000
Deferred State and Local Income Tax Expense (Benefit)				6,000,000.0	3,600,000		14,500,000
Deferred Change In Unrecognized Tax Expense (Benefit)				700,000	(2,000,000.0)		22,100,000
Deferred investment tax credits				(100,000)	(100,000)		(100,000)
Income Tax Expense (Benefit)				25,600,000	38,900,000		68,400,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items				52,700,000	44,200,000		(414,200,000)
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities				(23,800,000)	(22,000,000.0)		540,700,000
Other Comprehensive Income (Loss), Tax				100,000	(100,000)		0
Deferred Income Tax Expense (Benefit)				$ 29,000,000.0	$ 22,100,000		$ 126,500,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment										758,700,000	680,600,000	[3]
Deferred Tax Liabilities, Operating Lease Asset										115,800,000	0	[3]
Deferred Tax Liabilities, Regulatory Assets										49,700,000	49,200,000	[3]
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits										33,100,000	32,300,000	[3]
Deferred Tax Liabilities, Gross										957,300,000	762,100,000	[3]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Regulatory Liabilities										111,200,000	116,800,000	[3]
Deferred Tax Assets, Operating Lease Liabilities										115,800,000	0	[3]
Deferred Tax Assets Unbilled Revenue Fuel Costs										18,300,000	12,700,000	[3]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits										5,800,000	5,600,000	[3]
Deferred Tax Assets, Operating Loss Carryforwards										100,000	200,000	[3]
Deferred Tax Assets, Tax Credit Carryforwards										29,500,000	5,700,000	[3]
Deferred Tax Assets, Other										4,800,000	2,000,000.0	[3]
Deferred Tax Assets, Net of Valuation Allowance										285,500,000	143,000,000.0	[3]
Deferred Tax Liabilities, Net										671,800,000	619,100,000	[3]
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Tax Credit Carryforward, Amount										29,500,000	5,700,000
Potential Tax Adjustments			$ 0
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards										$ 1,200,000	$ 2,900,000
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		426,000,000
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		28,000,000
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)		$ 45,000,000

[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.