Commitments and Contingencies, Purchased Power Agreements (Details) - Independent Power Producing Entities - MW	9 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	897	827
Purchase Power Agreement Expiration (year)	2041

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

No definition available.

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