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Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]  
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences:
 
 
Nine Months Ended Sept. 30,
 
 
2018
 
2017
Federal statutory rate
 
21.0
 %
 
35.0
 %
State tax (net of federal tax effect)
 
2.3

 
2.3

Increases (decreases) in tax from:
 

 

Regulatory differences - ARAM (a)
 
(4.0
)
 
—

Regulatory differences - ARAM deferral (b)
 
1.7

 
—

Regulatory differences - reversal of prior quarters' ARAM deferral (b)
 
(0.2
)
 
—

Regulatory differences - other utility plant items
 
(1.3
)
 
(0.8
)
Tax credits (net of federal income tax expense)
 
(0.7
)
 
(0.7
)
Other (net)
 
(1.8
)
 
0.1

Effective income tax rate
 
17.0
 %
 
35.9
 %
(a)  
The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.
(b)
ARAM has been deferred when regulatory treatment has not been established. As SPS 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.
Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block]
SPS is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows:
Tax Year(s)
 
Expiration
2009 - 2014
 
October 2019
2015
 
September 2019
2016
 
September 2020
Reconciliation of Unrecognized Tax Benefits
A reconciliation of the amount of unrecognized tax benefit is as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Unrecognized tax benefit — Permanent tax positions
 
$
2.8

 
$
2.3

Unrecognized tax benefit — Temporary tax positions
 
1.6

 
2.0

Total unrecognized tax benefit
 
$
4.4

 
$
4.3
Tax Benefits Associated with NOL and Tax Credit Carryforwards
The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
NOL and tax credit carryforwards
 
$
(6.9
)
 
$
(5.9
)