Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019		Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Tax Audits [Abstract]
Tax Credit Carryforward, Amount						$ 83.0		$ 30.0
Potential Federal Tax Adjustments		$ 0.0
Deferred Fuel Cost						9.0		18.0	[1]
Tax Benefits Associated With Nol And Tax Credit Carryforwards						6.0		4.0
Payable for interest related to unrecognized tax benefits						(1.0)	$ 1.0	1.0		$ 1.0	$ 1.0	$ 1.0
Unrecognized Tax Benefits, Interest on Income Taxes Expense		(2.0)	$ 0.0		$ 0.0
Unrecognized Tax Benefits, Income Tax Penalties Expense		$ 0.0	$ 0.0		$ 0.0
Deferred Tax Assets, Operating Loss Carryforwards, State and Local						1.0		1.0
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%		21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		2.30%	2.20%		2.30%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		18.30%	7.90%		0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	[2]	(6.40%)	(5.00%)		(4.80%)
Amortization of excess nonplant deferred taxes		(0.80%)	(0.90%)		(1.20%)
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent		(0.70%)	(0.60%)		(0.70%)
Effective Income Tax Reconciliation, Adjustments Attributable to Tax Returns, Percent		(0.60%)	(0.10%)		(1.50%)
Effective Income Tax Rate Reconcilation, Change in unrecognized tax benefits, percent		0.30%	0.20%		0.10%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.30%)	0.10%		0.30%
Effective Income Tax Rate Reconciliation, Percent		(3.50%)	9.00%		15.50%
Deferred tax expense (benefit) excluding selected items		$ 53.0	$ 53.0		$ 44.0
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(31.0)	(24.0)		(22.0)
Income Tax Expense (Benefit)		(10.0)	26.0		39.0
Present value of minimum obligation						491.0
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions						3.0		3.0
Unrecognized tax benefit — Temporary tax positions						4.0		2.0
Total unrecognized tax benefit		5.0	5.0		5.0	7.0	5.0	5.0		5.0	5.0	4.0
Unrecognized Tax Benefits That Would Not Impact Effective Tax Rate						4.0		2.0
Unrecognized Tax Benefits		5.0	5.0		5.0	7.0	$ 5.0	5.0		$ 5.0	5.0	$ 4.0
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1		5.0	5.0
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		1.0	0.0		1.0
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		5.0	0.0		0.0
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions		(4.0)	0.0		0.0
Balance at Dec. 31		$ 7.0	$ 5.0		$ 5.0
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards						(6.0)		(4.0)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards						(2.0)		(1.0)
Decrease in Unrecognized Tax Benefits is Reasonably Possible						5.0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%		21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		2.30%	2.20%		2.30%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(18.30%)	(7.90%)		0.00%
Effective Income Tax Reconciliation, Adjustments Attributable to Tax Returns, Percent		(0.60%)	(0.10%)		(1.50%)
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	[2]	6.40%	5.00%		4.80%
Amortization of excess nonplant deferred taxes		0.80%	0.90%		1.20%
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent		(0.70%)	(0.60%)		(0.70%)
Effective Income Tax Rate Reconciliation, Percent		(3.50%)	9.00%		15.50%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Income Tax Expense (Benefit)		$ (10.0)	$ 26.0		$ 39.0
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items		53.0	53.0		44.0
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		$ (31.0)	$ (24.0)		$ (22.0)
Deferred Tax Assets, Tax Deferred Expense						22.0		29.0			22.0
Deferred Tax Assets, Gross [Abstract]
Effective Income Tax Rate Reconcilation, Change in unrecognized tax benefits, percent		0.30%	0.20%		0.10%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.30%)	0.10%		0.30%
Deferred Tax Assets, Tax Deferred Expense						22.0		29.0			$ 22.0
Present value of minimum obligation						491.0
income tax expense [Member]
Tax Audits [Abstract]
Current Federal Tax Expense (Benefit)		$ (31.0)	$ (4.0)		$ 12.0
Current State and Local Tax Expense (Benefit)		(1.0)	1.0		3.0
Current Change In Unrecognized Tax Expense (Benefit)		0.0	0.0		2.0
Deferred Federal Income Tax Expense (Benefit)		13.0	22.0		20.0
Deferred State and Local Income Tax Expense (Benefit)		8.0	6.0		4.0
Deferred Change In Unrecognized Tax Expense (Benefit)		1.0	1.0		(2.0)
Income Tax Expense (Benefit)		(10.0)	26.0		39.0
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)		(31.0)	(4.0)		12.0
Current State and Local Tax Expense (Benefit)		(1.0)	1.0		3.0
Current Change In Unrecognized Tax Expense (Benefit)		0.0	0.0		2.0
Deferred Federal Income Tax Expense (Benefit)		13.0	22.0		20.0
Deferred State and Local Income Tax Expense (Benefit)		8.0	6.0		4.0
Deferred Change In Unrecognized Tax Expense (Benefit)		1.0	1.0		(2.0)
Income Tax Expense (Benefit)		(10.0)	26.0		$ 39.0
Net Deferred Tax Liablility [Member]
Tax Audits [Abstract]
Tax Credit Carryforward, Amount						83.0		30.0	[1]
Differences between book and tax basis of property		838.0	759.0	[1]
Operating Lease Assets						109.0		116.0	[1]
Regulatory Assets						59.0		50.0	[1]
Deferred tax liability - Pension expense						33.0		33.0	[1]
Deferred Tax Assets, Tax Deferred Expense, Other						2.0		0.0	[1]
Deferred Tax Liabilities, Tax Deferred Income						1,041.0		958.0	[1]
Present value of minimum obligation						109.0		116.0	[1]
Regulatory Liabilities						104.0		111.0	[1]
other employee benefits						7.0		6.0	[1]
Deferred Tax Assets, Other						4.0		5.0	[1]
Deferred Tax Liabilities, Net						725.0		672.0	[1]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Tax Assets, Tax Deferred Expense						316.0		286.0	[1]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Other						4.0		5.0	[1]
Deferred Tax Liabilities, Net						725.0		672.0	[1]
Deferred Tax Assets, Tax Deferred Expense						316.0		286.0	[1]
Differences between book and tax basis of property		838.0	$ 759.0	[1]
Operating Lease Assets						109.0		116.0	[1]
Regulatory Assets						59.0		50.0	[1]
Deferred tax liability - Pension expense						33.0		33.0	[1]
Deferred Tax Assets, Tax Deferred Expense, Other						2.0		0.0	[1]
Deferred Tax Liabilities, Tax Deferred Income						1,041.0		958.0	[1]
Present value of minimum obligation						109.0		116.0	[1]
other employee benefits						7.0		6.0	[1]
Regulatory Liabilities						$ 104.0		$ 111.0	[1]
Xcel Energy [Member]
Tax Audits [Abstract]
Potential Tax Adjustments		$ 0.0

[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.