Fair Value of Financial Assets and Liabilities - Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Derivatives, Fair Value
Return Cash Collateral		$ 0		$ 0		$ 0
Changes in Level 3 Commodity Derivatives
Derivative instruments designated as fair value hedges		0	$ 0	0	$ 0
Commodity Contract [Member]
Changes in Level 3 Commodity Derivatives
Balance at beginning of period		184	146	67	39
Purchases	[1]	0	2	199	107
Settlements	[1]	(39)	(64)	(161)	(183)
Net gains recognized as regulatory assets and liabilities	[1]	(5)	28	35	149
Balance at end of period		140	$ 112	140	$ 112
Fair Value Measured on a Recurring Basis
Derivatives, Fair Value
Derivative asset, gross		140		140		67
Asset, netting	[2]	0		0		0
Changes in Level 3 Commodity Derivatives
Derivative Asset, Current		140		140		67
Fair Value Measured on a Recurring Basis | Level 1
Derivatives, Fair Value
Derivative asset, gross		0		0		0
Fair Value Measured on a Recurring Basis | Level 2
Derivatives, Fair Value
Derivative asset, gross		0		0		0
Fair Value Measured on a Recurring Basis | Level 3
Derivatives, Fair Value
Derivative asset, gross		140		140		67
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		140		140		67
Asset, netting	[2]	0		0		0
Changes in Level 3 Commodity Derivatives
Derivative Asset, Current		140		140		67
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Level 1 | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Level 2 | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Level 3 | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		$ 140		$ 140		$ 67

[1] Relates primarily to FTR instruments administered by SPP.

[2] SPS nets derivative instruments and related collateral on its balance sheet when supported by a legally enforceable master netting agreement. At Sept. 30, 2025 and Dec. 31, 2024, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. Counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.