Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details)	Jun. 30, 2014 Greenhouse Gas New Source Performance Standard for Modified and Reconstructed Power Plants	Apr. 30, 2014 Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Percentage of a comparable new plant's capital cost which would have to be exceeded to consider a project as a reconstruction under the proposed GHG NSPS for Modified and Reconstructed Power Plants (in hundredths)	50.00%
Number of issues on which the D.C. Circuit overturned the CSAPR		2

Percentage of a comparable new power plant's capital cost which would have to be exceeded to consider a project as a reconstruction under the proposed GHG NSPS for Modified and Reconstructed Power Plants

No definition available.

Name:	sps_PercentageofacomparablenewplantscapitalcostwhichwouldhavetobeexceededtoconsideraprojectasareconstructionundertheproposedGHGNSPSforModifiedandReconstructedPowerPlants
Namespace Prefix:	sps_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant