Commitments and Contingencies, Purchased Power Agreements (Details) (Independent Power Producing Entities [Member])	Sep. 30, 2012 MW	Dec. 31, 2011 MW
Independent Power Producing Entities [Member]
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	827	827

Generating capacity under long-term purchased power agreements.

No definition available.

Name:	sps_GeneratingCapacityUnderLongTermPurchasedPowerAgreements
Namespace Prefix:	sps_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant