Commitments and Contingencies, Purchased Power Agreements (Details) - Independent Power Producing Entities - MW	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	967	897
Purchase Power Agreement Expiration (year)	2041

The expiration of purchase power agreements currently under contract (year).

No definition available.

Name:	sps_PurchasePowerAgreementExpiration
Namespace Prefix:	sps_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration