Commitments and Contingencies, Purchased Power Agreements (Details) - Independent Power Producing Entities - MW	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	827	827
Purchase Power Agreement Duration, Maximum	2033	2033

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

No definition available.

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