Benefit Plans and Other Postretirement Benefits (Details) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
		Jan. 31, 2019 USD ($) Plan	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Components of Net Periodic Benefit Cost [Abstract]
Operating and maintenance expenses			$ 47.5	$ 50.0	$ 150.8	$ 149.3
Pension Plan [Member]
Components of Net Periodic Benefit Cost [Abstract]
Service cost			1.1	1.2	3.3	3.6
Interest cost (a)	[1]		1.4	1.4	4.3	4.1
Expected return on plan assets (a)	[1]		(2.0)	(2.3)	(6.2)	(6.8)
Amortization of prior service cost (credit) (a)	[1]		0.0	0.0	0.0	0.0
Amortization of net loss (a)	[1]		1.1	1.4	3.3	4.2
Settlement charge (b)	[2]		0.0	5.2	0.0	5.2
Net periodic benefit cost			1.6	6.9	4.7	10.3
Credits (costs) not recognized due to the effects of regulation			0.0	(3.1)	0.2	(2.8)
Net benefit cost recognized for financial reporting			1.6	3.8	4.9	7.5
Operating and maintenance expenses						1.2
Contributions to pension plans		$ 7.2
Pension Plan [Member] | Xcel Energy Inc.
Components of Net Periodic Benefit Cost [Abstract]
Contributions to pension plans		$ 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 [Abstract]
Service cost			0.0	0.0	0.0	0.0
Interest cost (a)	[1]		0.1	0.2	0.4	0.4
Expected return on plan assets (a)	[1]		0.0	0.0	0.0	0.0
Amortization of prior service cost (credit) (a)	[1]		(0.1)	(0.1)	(0.2)	(0.2)
Amortization of net loss (a)	[1]		0.1	0.1	0.2	0.4
Settlement charge (b)	[2]		0.0	0.0	0.0	0.0
Net periodic benefit cost			0.1	0.2	0.4	0.6
Credits (costs) not recognized due to the effects of regulation			0.0	0.0	0.0	0.0
Net benefit cost recognized for financial reporting			$ 0.1	$ 0.2	$ 0.4	$ 0.6

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

[2] A settlement charge is required when the amount of all lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter of 2018 as a result of lump-sum distributions during the 2018 plan year, NSP-Wisconsin recorded a total pension settlement charge of $5.2 million, the majority of which was not recognized due to the effects of regulation. A total of $1.2 million of that amount was recorded in other expense in the third quarter of 2018.