Income Taxes (Details) - USD ($)		3 Months Ended				9 Months Ended	12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2015	Dec. 31, 2018	Dec. 31, 2017
Income Tax Examination [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		3.70%		3.70%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(7.50%)		(0.00%)
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	[1]	(3.90%)		(1.50%)
Regulatory differences - reversal of prior quarters' ARAM deferral (b)		(1.20%)		(1.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		0.00%		0.40%
Effective Income Tax Rate Reconciliation, Percent		12.10%		22.60%
Tax Audits [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		$ (800,000)	$ (700,000)				$ (700,000)	$ (300,000)
Unrecognized Tax Benefits, Interest on Income Taxes Expense		(100,000)					(400,000)
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions		5,700,000	5,400,000				5,400,000
Unrecognized tax benefit — Temporary tax positions		4,800,000	4,900,000				4,900,000
Total unrecognized tax benefit		10,500,000	10,300,000				10,300,000
NOL and tax credit carryforwards		(6,500,000)	(5,600,000)				(5,600,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards		(3,000,000)	(2,000,000)				(2,000,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		8,700,000
Amounts accrued for penalties related to unrecognized tax benefits		0	$ 0				$ 0
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination			2014 - 2016			2012 and 2013
Potential Tax Adjustments		$ 0
Tax years under examination, Concluded					2012 and 2013
State Jurisdiction (Colorado)
Tax Audits [Abstract]
Earliest year subject to examination		2009

[1] Regulatory differences for income tax purposes primarily include the ARAM, ARAM deferral and AFUDC - Equity. ARAM is a method to flow back excess deferred taxes to customers. ARAM has been deferred when regulatory treatment has not been established. As Xcel Energy received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.