Commitments and Contingencies - Variable Interest Entities (Details) - MW	Sep. 30, 2020	Dec. 31, 2019
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	1,197	1,197

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