Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) - Capital Addition Purchase Commitments - Regional Haze Rules $ in Millions	Mar. 31, 2017 USD ($)
Harrington Units 1 and 2 [Member]
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation	$ 400
Tolk Units 1 and 2 [Member]
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation	$ 600

Estimated undiscounted amount of the accrual required to comply with required regulation.

No definition available.

Name:	sps_LiabilityForEstimatedCostToComplyWithRegulation
Namespace Prefix:	sps_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant