Benefit Plans and Other Postretirement Benefits (Details) $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended
		Jul. 01, 2019 USD ($)	Jan. 31, 2019 USD ($) Plan	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Pension Plan [Member]
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost				$ 6.4	$ 7.3	$ 19.2	$ 21.8
Interest cost	[1]			12.9	11.8	38.7	35.5
Expected return on plan assets	[1]			(17.1)	(17.1)	(51.4)	(51.4)
Amortization of prior service credit	[1]			(0.8)	(0.9)	(2.5)	(2.5)
Amortization of net loss	[1]			6.3	7.8	19.1	23.4
Net periodic benefit cost (credit)				7.7	8.9	23.1	26.8
Credits (costs) not recognized due to the effects of regulation				0.8	(4.1)	4.4	(1.7)
Net benefit cost (credit) recognized for financial reporting				8.5	4.8	27.5	25.1
Contributions to pension plans		$ 2.7	$ 43.0
Pension Plan [Member] | Xcel Energy Inc.
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Contributions to pension plans		$ 4.0	$ 150.0
Number of Xcel Energy's pension plans to which contributions were made | Plan			4
Other Postretirement Benefits Plan [Member]
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost				0.1	0.2	0.4	0.5
Interest cost	[1]			3.9	3.8	11.7	11.2
Expected return on plan assets	[1]			(4.7)	(5.7)	(14.2)	(17.0)
Amortization of prior service credit	[1]			(1.3)	(1.6)	(4.1)	(4.7)
Amortization of net loss	[1]			0.7	1.0	2.2	3.1
Net periodic benefit cost (credit)				(1.3)	(2.3)	(4.0)	(6.9)
Credits (costs) not recognized due to the effects of regulation				0.4	1.4	0.9	1.4
Net benefit cost (credit) recognized for financial reporting				$ (0.9)	$ (0.9)	$ (3.1)	$ (5.5)

[1] The components of net periodic cost other than the service cost component are included in the line item “other income, net” in the consolidated statement of income or capitalized on the consolidated balance sheet as a regulatory asset.