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Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2025

Fair Value Disclosures [Abstract]

Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis

The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2025 and 2024:

						December 31, 2025																		December 31, 2024
						Level 1			Level 2			Level 3			Total									Level 1			Level 2			Level 3			Total
Assets:
Ameren Missouri
			Derivative assets – commodity contracts:
			Natural gas			—			1			—			1									—			4			—			4
			Power			—			—			11			11									—			—			6			6
			Total derivative assets – commodity contracts			$	—		$	1		$	11		$	12								$	—		$	4		$	6		$	10
			Nuclear decommissioning trust fund:
			Equity securities:
			U.S. large capitalization			$	1,028		$	—		$	—		$	1,028								$	911		$	—		$	—		$	911
			Debt securities:
			U.S. Treasury and agency securities			—			225			—			225									—			191			—			191
			Corporate bonds			—			177			—			177									—			145			—			145
			Other			—			84			—			84									—			86			—			86
			Total nuclear decommissioning trust fund			$	1,028		$	486		$	—		$	1,514		(a)						$	911		$	422		$	—		$	1,333		(a)
			Total Ameren Missouri			$	1,028		$	487		$	11		$	1,526								$	911		$	426		$	6		$	1,343
Ameren Illinois
			Derivative assets – commodity contracts:
			Natural gas			$	—		$	2		$	2		$	4								$	—		$	3		$	3		$	6
			Total Ameren Illinois			$	—		$	2		$	2		$	4								$	—		$	3		$	3		$	6
Ameren
			Derivative assets – commodity contracts(b)			$	—		$	3		$	13		$	16								$	—		$	7		$	9		$	16
			Nuclear decommissioning trust fund(c)			1,028			486			—			1,514			(a)						911			422			—			1,333			(a)
			Total Ameren			$	1,028		$	489		$	13		$	1,530								$	911		$	429		$	9		$	1,349
Liabilities:
Ameren Missouri
			Derivative liabilities – commodity contracts:
			Fuel oils			$	5		$	—		$	—		$	5								$	4		$	—		$	—		$	4
			Natural gas			—			10			—			10									—			11			—			11
			Total Ameren Missouri			$	5		$	10		$	—		$	15								$	4		$	11		$	—		$	15
Ameren Illinois
			Derivative liabilities – commodity contracts:
			Natural gas			$	1		$	28		$	3		$	32								$	1		$	28		$	6		$	35
			Power			—			13			66			79									—			—			53			53
			Total Ameren Illinois			$	1		$	41		$	69		$	111								$	1		$	28		$	59		$	88
Ameren
			Derivative liabilities – commodity contracts(b)			$	6		$	51		$	69		$	126								$	5		$	39		$	59		$	103

(a)Balance excludes $12 million and $9 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2025 and 2024, respectively.

(b)See the Ameren Missouri and Ameren Illinois sections of the table for the fair value of Ameren’s derivative assets and liabilities by type of commodity.

(c)See the Ameren Missouri section of the table for Ameren’s nuclear decommissioning trust fund by investment type.

Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy

The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the years ended December 31, 2025 and 2024:

			2025															2024
			Ameren Missouri			Ameren Illinois			Ameren									Ameren Missouri			Ameren Illinois			Ameren
Beginning balance at January 1			$	6		$	(53)		$	(47)								$	4		$	(68)		$	(64)
Realized and unrealized gains (losses) included in regulatory assets/liabilities			29			(24)			5									12			(1)			11
Settlements			(24)			11			(13)									(10)			16			6
Ending balance at December 31			$	11		$	(66)		$	(55)								$	6		$	(53)		$	(47)
Change in unrealized gains (losses) related to assets/liabilities held at December 31			11			(23)			(12)									6			3			9

Fair Value Inputs, Assets and Liabilities, Quantitative Information

The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of December 31, 2025 and 2024:

						Fair Value																		Weighted Average(b)
			Commodity			Assets			Liabilities						Valuation Technique(s)			Unobservable Input(a)			Range
2025			Power(c)			$	11		$	(66)					Discounted cash flow			Average forward peak and off-peak pricing – forwards/swaps ($/MWh)			33 – 72			43
																		Nodal basis ($/MWh)			(9) – (2)			(5)
2024			Power(c)			$	6		$	(53)					Discounted cash flow			Average forward peak and off-peak pricing – forwards/swaps ($/MWh)			32 – 69			45
																		Nodal basis ($/MWh)			(8) – (2)			(5)

(a)Generally, significant increases (decreases) in these inputs in isolation would result in a significantly higher (lower) fair value measurement.

(b)Unobservable inputs were weighted by relative fair value.

(c)Valuations use visible forward prices adjusted for nodal-to-hub basis differentials.

Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]

The following table sets forth the carrying amount and, by level within the fair value hierarchy, the fair value of long-term debt (including current portion), disclosed, but not recorded, at fair value as of December 31, 2025 and 2024:

			Carrying Amount(a)									Fair Value
Long-Term Debt (Including Current Portion):															Level 2						Level 3						Total
			December 31, 2025
Ameren(b)			$	19,187											$	17,433					$	559		(c)			$	17,992
Ameren Missouri(d)			8,230												7,608						—						7,608
Ameren Illinois(d)			6,259												5,753						—						5,753
			December 31, 2024
Ameren(b)			$	17,579											$	15,395					$	538		(c)			$	15,933
Ameren Missouri(d)			7,745												6,926						—						6,926
Ameren Illinois(d)			5,852												5,243						—						5,243

(a)Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $136 million, $62 million, and $56 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2025. Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $129 million, $62 million, and $51 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2024.

(b)Amount excludes Ameren (parent)’s repurchase of Ameren Missouri’s senior secured notes and first mortgage bonds and Ameren Illinois’ first mortgage bonds that were accounted for as a debt extinguishment. See Note 5 – Long-term Debt and Equity Financings for additional information.

(c)The Level 3 fair value amount consists of ATXI’s senior unsecured notes.

(d)Amount includes Ameren Missouri’s senior secured notes and first mortgage bonds and Ameren Illinois’ first mortgage bonds that were repurchased by Ameren (parent) in 2025 and 2024.

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