Fair Value of Financial Assets and Liabilities - Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2026		Mar. 31, 2025	Dec. 31, 2025
Derivatives, Fair Value
Return Cash Collateral		$ 0			$ 0
Changes in Level 3 Commodity Derivatives
Derivative instruments designated as fair value hedges		0		$ 0
Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		112			79
Asset, netting	[1]	0			0
Derivative Asset, Current		112			79
Derivative Asset, Noncurrent		20	[2]		0
Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		20	[2]		0
Asset, netting	[1]	0	[2]		0
Commodity Contract [Member]
Changes in Level 3 Commodity Derivatives
Balance at beginning of period		79		67
Purchases	[3]	105		66
Settlements	[3]	(23)		(48)
Net gains recognized as regulatory assets and liabilities	[3]	(29)		18
Balance at end of period		132		$ 103
Level 1 | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		0			0
Level 1 | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		0	[2]		0
Level 2 | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		0			0
Level 2 | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		0	[2]		0
Level 3 | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		112			79
Level 3 | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		20	[2]		0
Other Derivative Instruments | Electric Commodity | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		112			79
Asset, netting	[1]	0			0
Derivative Asset, Current		112			79
Derivative Asset, Noncurrent		20			0
Other Derivative Instruments | Electric Commodity | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		20			0
Asset, netting	[1]	0			0
Other Derivative Instruments | Level 1 | Electric Commodity | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		0			0
Other Derivative Instruments | Level 1 | Electric Commodity | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		0			0
Other Derivative Instruments | Level 2 | Electric Commodity | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		0			0
Other Derivative Instruments | Level 2 | Electric Commodity | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		0			0
Other Derivative Instruments | Level 3 | Electric Commodity | Fair Value, Recurring [Member]
Derivatives, Fair Value
Derivative asset, gross		112			79
Other Derivative Instruments | Level 3 | Electric Commodity | Fair Value, Recurring [Member] | Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, gross		$ 20			$ 0

[1] SPS nets derivative instruments and related collateral on its balance sheet when supported by a legally enforceable master netting agreement. At March 31, 2026 and Dec. 31, 2025, 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.

[2] Noncurrent derivative assets are included in other as of March 31, 2026

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