Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2012

Fair Value Measurements [Abstract]

Schedule of Valuation Process And Unobservable Inputs

		Fair Value		Valuation Technique(s)	Unobservable Input	Range [Weighted Average]
Level 3 Derivative assets - commodity contracts(b):
Ameren(a)	Fuel oils	$	9	Escalated exchange settled pricing	Escalation rate(c)	0.68% - 0.71% [0.71%]
				Option model	Volatilities(c)	23% - 28% [25%]
				Credit risk discount	Counterparty credit risk(d)	0.12% - 12% [1%]
	Power(e)		170	Option model	Volatilities(d)	15% - 68% [19%]
					Average bid/ask consensus pricing(d)	$16/MWh-$39/MWh [$35/MWh]
				Power forwards/swaps third party pricing	Average bid/ask consensus pricing(d)	$20/MWh - $49/MWh [$29/MWh]
				FTR third party pricing	Estimated auction price(c)	$(1,569)/MW - $3,019/MW [$173/MW]
				Basis to nodal valuation price	Nodal basis(c)	$(6)/MWh - $(0.20)/MWh [$(3)/MWh]
				Credit risk discount	Counterparty credit risk(d)	0.06% - 13% [5%]
Ameren Missouri	Fuel oils	$	7	Escalated exchange settled pricing	Escalation rate(c)	0.68% - 0.71% [0.71%]
				Option model	Volatilities(c)	23% - 28% [25%]
				Credit risk discount	Counterparty credit risk(d)	0.12% - 4% [1%]
	Power(e)		28	Option model	Volatilities(d)	40% - 68% [61%]
					Average bid/ask consensus pricing(d)	$16/MWh - $31/MWh [$19/MWh]
				Power forwards/swaps third party pricing	Average bid/ask consensus pricing(d)	$17/MWh - $49/MWh [$23/MWh]
				FTR third party pricing	Estimated auction price(c)	$(1,569)/MW - $3,019/MW [$170/MW]
				Basis to nodal valuation price	Nodal basis(c)	$(3)/MWh - $(0.48)/MWh [$(2)/MWh]
				Credit risk discount	Counterparty credit risk(d)	0.06% - 12% [5%]
Genco	Fuel oils	$	2	Escalated exchange settled pricing	Escalation rate(c)	0.68% - 0.71% [0.71%]
				Option model	Volatilities(c)	25% - 28% [26%]
				Credit risk discount	Counterparty credit risk(d)	0.12% - 12% [2%]
Level 3 Derivative liabilities - commodity contracts(b):
Ameren(a)	Power(e)	$	194	Option model	Volatilities(d)	15% - 40% [24%]
					Average bid/ask consensus pricing(d)	$16/MWh - $39/MWh [$34/MWh]
				Power forwards/swaps third party pricing	Average bid/ask consensus pricing(d)	$20/MWh - $49/MWh [$28/MWh]

		Fair Value		Valuation Technique(s)	Unobservable Input	Range [Weighted Average]
				Basis to nodal valuation price	Nodal basis(c)	$(6)/MWh - $(0.20)/MWh [$(3)/MWh]
				Power market simulation model	Estimated future gas prices(c)	$4/mmbtu - $6/mmbtu [$5/mmbtu]
				Contract price allocation	Estimated renewable energy credit costs(c)	$5/credit - $7/credit [$6/credit]
				Credit risk discount	Ameren credit risk(d)	3% - 6% [6%]
	Uranium		1	Third party pricing	Average bid/ask consensus pricing(c)	$51/pound - $55/pound [$52/pound]
Ameren Missouri	Power(e)	$	8	Option model	Volatilities(d)	35% - 40% [37%]
					Average bid/ask consensus pricing(d)	$16/MWh - $27/MWh [$23/MWh]
				Power forwards/swaps third party pricing	Average bid/ask consensus pricing(d)	$20/MWh - $49/MWh [$27/MWh]
				Basis to nodal valuation price	Nodal basis(c)	$(3)/MWh - $(0.48)/MWh [$(2)/MWh]
				Credit risk discount	Ameren Missouri credit risk(d)	3%
	Uranium		1	Third party pricing	Average bid/ask consensus pricing(c)	$51/pound - $55/pound [$52/pound]
Ameren Illinois	Power(e)	$	284	Power forwards/swaps third party pricing	Average bid/ask consensus pricing(c)	$20/MWh - $36/MWh $[28/MWh]
				Basis to nodal valuation price	Nodal basis(d)	$(4)/MWh - $(1)/MWh [$(3)/MWh]
				Power market simulation model	Estimated future gas prices(c)	$4/mmbtu - $6/mmbtu [$5/mmbtu]
				Contract price allocation	Estimated renewable energy credit costs(c)	$5/credit - $7/credit [$6/credit]
				Credit risk discount	Ameren Illinois credit risk(d)	6%

(a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

(b) The derivative asset and liability balances are presented net of counterparty credit considerations.

(c) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

(d) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.

(e) Power valuations utilize visible third party pricing evaluated by month for peak and off-peak through 2015. Valuations beyond 2015 utilize power market simulation modeled pricing by month for peak and off-peak.

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

		Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Assets:
Ameren(a)	Derivative assets - commodity contracts(b):
	Fuel oils	$	36	$	-	$	9	$	45
	Natural gas		4		2		-		6
	Power		-		15		170		185
	Nuclear Decommissioning Trust Fund(c):
	Cash and cash equivalents		3		-		-		3
	Equity securities:
	U.S. large capitalization		264		-		-		264
	Debt securities:
	Corporate bonds		-		46		-		46
	Municipal bonds		-		1		-		1
	U.S. treasury and agency securities		-		66		-		66
	Asset-backed securities		-		10		-		10
	Other		-		1		-		1
Ameren	Derivative assets - commodity contracts(b):
Missouri	Fuel oils		20		-		7		27
	Natural gas		2		-		-		2
	Power		-		7		28		35
	Nuclear Decommissioning Trust Fund(c):
	Cash and cash equivalents		3		-		-		3
	Equity securities:
	U.S. large capitalization		264		-		-		264
	Debt securities:
	Corporate bonds		-		46		-		46
	Municipal bonds		-		1		-		1
	U.S. treasury and agency securities		-		66		-		66
	Asset-backed securities		-		10		-		10
	Other		-		1		-		1
Ameren	Derivative assets - commodity contracts(b):
Illinois	Natural gas		-		1		-		1
Genco	Derivative assets - commodity contracts(b):
	Fuel oils		11		-		2		13
	Natural gas		2		-		-		2
Liabilities:
Ameren(a)	Derivative liabilities - commodity contracts(b):
	Coal	$	4	$	-	$	-	$	4
	Fuel oils		1		-		-		1
	Natural gas		19		196		-		215
	Power		-		16		194		210
	Uranium		-		-		1		1
Ameren	Derivative liabilities - commodity contracts(b):
Missouri	Fuel oils		1		-		-		1
	Natural gas		12		16		-		28
	Power		-		7		8		15
	Uranium		-		-		1		1
Ameren	Derivative liabilities - commodity contracts(b):
Illinois	Natural gas		5		179		-		184
	Power		-		-		284		284
Genco	Derivative liabilities - commodity contracts(b):
	Coal		3		-		-		3
	Natural gas		1		-		-		1

(a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

(b) The derivative asset and liability balances are presented net of counterparty credit considerations.

(c) Balance excludes $(1) million of receivables, payables, and accrued income, net.

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, 2011:

		Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Assets:
Ameren(a)	Derivative assets - commodity contracts(b):
	Fuel oils	$	33	$	-	$	4	$	37
	Natural gas		4		-		2		6
	Power		-		2		193		195
	Nuclear Decommissioning Trust Fund(c):
	Cash and cash equivalents		3		-		-		3
	Equity securities:
	U.S. large capitalization		234		-		-		234
	Debt securities:
	Corporate bonds		-		44		-		44
	Municipal bonds		-		1		-		1
	U.S. treasury and agency securities		-		65		-		65
	Asset-backed securities		-		10		-		10
	Other		-		1		-		1
Ameren	Derivative assets - commodity contracts(b):
Missouri	Fuel oils		20		-		3		23
	Natural gas		2		-		-		2
	Power		-		1		29		30
	Nuclear Decommissioning Trust Fund(c):
	Cash and cash equivalents		3		-		-		3
	Equity securities:
	U.S. large capitalization		234		-		-		234
	Debt securities:
	Corporate bonds		-		44		-		44
	Municipal bonds		-		1		-		1
	U.S. treasury and agency securities		-		65		-		65
	Asset-backed securities		-		10		-		10
	Other		-		1		-		1
Ameren	Derivative assets - commodity contracts(b):
Illinois	Natural gas		-		-		2		2
	Power		-		-		77		77
Genco	Derivative assets - commodity contracts(b):
	Fuel oils		10		-		1		11
	Natural gas		2		-		-		2
Liabilities:
Ameren(a)	Derivative liabilities - commodity contracts(b):
	Fuel oils	$	2	$	-	$	-	$	2
	Natural gas		22		-		176		198
	Power		-		2		78		80
	Uranium		-		-		1		1
Ameren	Derivative liabilities - commodity contracts(b):
Missouri	Fuel oils		1		-		-		1
	Natural gas		12		-		14		26
	Power		-		1		8		9
	Uranium		-		-		1		1
Ameren	Derivative liabilities - commodity contracts(b):
Illinois	Natural gas		7		-		162		169
	Power		-		-		217		217
Genco	Derivative liabilities - commodity contracts(b):
	Fuel oils		1		-		-		1
	Natural gas		2		-		-		2

(a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

(b) The derivative asset and liability balances are presented net of counterparty credit considerations.

(c) Balance excludes $(1) million of receivables, payables, and accrued income, net.

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

	Net derivative commodity contracts
	Ameren Missouri			Ameren Illinois			Genco			Other(c)			Ameren
Fuel oils:
Beginning balance at January 1, 2012	$	3		$	(a	)	$	1		$	—		$	4
Realized and unrealized gains (losses):
Included in earnings(b)		-			(a	)		2			-			2
Included in regulatory assets/liabilities		2			(a	)		(a	)		(a	)		2
Total realized and unrealized gains (losses)		2			(a	)		2			-			4
Transfers into Level 3		2			(a	)		-			-			2
Transfers out of Level 3		-			(a	)		(1	)		-			(1	)
Ending balance at March 31, 2012	$	7		$	(a	)	$	2		$			$	9
Change in unrealized gains (losses) related to assets/liabilities held at March 31,2012	$	2		$	(a	)	$	1		$			$	3
Natural gas:
Beginning balance at January 1, 2012	$	(14	)	$	(160	)	$	-		$	-		$	(174	)
Realized and unrealized gains (losses):
Included in regulatory assets/liabilities		(2	)		(26	)		(a	)		(a	)		(28	)
Total realized and unrealized gains (losses)		(2	)		(26	)		(a	)		(a	)		(28	)
Settlements		1			16			-			-			17
Transfer out of Level 3		15			170			-			-			185
Ending balance at March 31, 2012	$	-		$	-		$	-		$	-		$	-
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2012	$	-		$	-		$	-		$	-		$	-
Power:
Beginning balance at January 1, 2012	$	21		$	(140	)	$	-		$	234		$	115
Realized and unrealized gains (losses):
Included in earnings(b)		-			-			-			8			8
Included in OCI		-			-			-			24			24
Included in regulatory assets/liabilities		13			(220	)		(a	)		49			(158	)
Total realized and unrealized gains (losses)		13			(220	)		-			81			(126	)
Purchases		-			-			-			(1	)		(1	)
Sales		-			-			-			1			1
Settlements		(13	)		76			-			(77	)		(14	)
Transfers out of Level 3		(1	)		-			-			2			1
Ending balance at March 31, 2012	$	20		$	(284	)	$	-		$	240		$	(24	)
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2012	$	10		$	(202	)(d)	$	-		$	59		$	(133	)
Uranium:
Beginning balance at January 1, 2012	$	(1	)	$	(a	)	$	(a	)	$	(a	)	$	(1	)
Realized and unrealized gains (losses):
Included in regulatory assets/liabilities		-			(a	)		(a	)		(a	)		-
Total realized and unrealized gains (losses)		-			(a	)		(a	)		(a	)		-
Ending balance at March 31, 2012	$	(1	)	$	(a	)	$	(a	)	$	(a	)	$	(1	)
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2012	$	-		$	(a	)	$	(a	)	$	(a	)	$	-

(a) Not applicable.

(b) Net gains and losses on fuel oils and natural gas derivative commodity contracts are recorded in "Operating Expenses - Fuel", while net gains and losses on power derivative commodity contracts are recorded in "Operating Revenues - Electric".

(c) Includes amounts for Merchant Generation nonregistrant subsidiaries and intercompany eliminations.

(d) The change in unrealized losses was due to decreases in long-term power prices applied to 20-year Ameren Illinois' swap contracts, which expire in May 2032.

The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of March 31, 2011:

	Net derivative commodity contracts
	Ameren Missouri			Ameren Illinois			Genco			Other(c)			Ameren
Fuel oils:
Beginning balance at January 1, 2011	$	30		$	(a	)	$	17		$	4		$	51
Realized and unrealized gains (losses):
Included in earnings(b)		-			(a	)		15			7			22
Included in regulatory assets/liabilities		31			(a	)		(a	)		(a	)		31
Total realized and unrealized gains (losses)		31			(a	)		15			7			53
Purchases		1			(a	)		-			-			1
Settlements		(5	)		(a	)		(3	)		(1	)		(9	)
Ending balance at March 31, 2011	$	57		$	(a	)	$	29		$	10		$	96
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2011	$	49		$	(a	)	$	16		$	4		$	69
Natural gas:
Beginning balance at January 1, 2011	$	(14	)	$	(134	)	$	-		$	-		$	(148	)

	Net derivative commodity contracts
	Ameren Missouri			Ameren Illinois			Genco			Other(c)			Ameren
Realized and unrealized gains (losses):
Included in regulatory assets/liabilities		-			7			(a	)		(a	)		7
Total realized and unrealized gains (losses)		-			7			-			-			7
Settlements		2			19			-			-			21
Ending balance at March 31, 2011	$	(12	)	$	(108	)	$	-		$	-		$	(120	)
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2011	$	1		$	6		$	-		$	-		$	7
Power:
Beginning balance at January 1, 2011	$	2		$	(352	)	$	3		$	383		$	36
Realized and unrealized gains (losses):
Included in earnings(b)		-			-			-			(3	)		(3	)
Included in OCI		-			-			-			-			-
Included in regulatory assets/liabilities		7			(30	)		(a	)		21			(2	)
Total realized and unrealized gains (losses)		7			(30	)		-			18			(5	)
Purchases		-			-			-			9			9
Sales		-			-			-			(9	)		(9	)
Settlements		(6	)		57			-			(51	)		-
Transfers into Level 3		(1	)		-			-			1			-
Ending balance at March 31, 2011	$	2		$	(325	)	$	3		$	351		$	31
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2011	$	3		$	(25	)	$	-		$	31		$	9
Uranium:
Beginning balance at January 1, 2011	$	2		$	(a	)	$	(a	)	$	(a	)	$	2
Realized and unrealized gains (losses):
Included in regulatory assets/liabilities		(1	)		(a	)		(a	)		(a	)		(1	)
Total realized and unrealized gains (losses)		(1	)		(a	)		(a	)		(a	)		(1	)
Ending balance at March 31, 2011	$	1		$	(a	)	$	(a	)	$	(a	)	$	1
Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2011	$	(1	)	$	(a	)	$	(a	)	$	(a	)	$	(1	)

(a) Not applicable.

(b) Net gains and losses on fuel oils and natural gas derivative commodity contracts are recorded in Operating Expenses - Fuel, while net gains and losses on power derivative commodity contracts are recorded in Operating Revenues - Electric.

(c) Includes amounts for Merchant Generation nonregistrant subsidiaries and intercompany eliminations.

Schedule Of Transfers Between Fair Value Hierarchy Levels

	2012			2011
Ameren - derivative commodity contracts:(a)
Transfers into Level 3 / Transfers out of Level 1 - Fuel oils	$	2		$		-
Transfers out of Level 3 / Transfers into Level 1 - Fuel oils		(1	)		-
Transfers out of Level 3 / Transfers into Level 2 - Natural gas		185			-
Transfers out of Level 3 / Transfers into Level 2 - Power		1			-
Net fair value of Level 3 transfers	$	187		$	-

	2012			2011
Ameren Missouri - derivative commodity contracts:
Transfers into Level 3 / Transfers out of Level 1 - Fuel oils	$	2		$	-
Transfers out of Level 3 / Transfers into Level 2 - Natural gas		15			-
Transfers into Level 3 / Transfers out of Level 2 - Power		-			(1	)
Transfers out of Level 3 / Transfers into Level 2 - Power		(1	)		-
Net fair value of Level 3 transfers	$	16		$	(1	)
Ameren Illinois - derivative commodity contracts:
Transfers out of Level 3 / Transfers into Level 2 - Natural gas	$	170		$	-
Genco - derivative commodity contracts:
Transfers out of Level 3 / Transfers into Level 1 - Fuel oils	$	(1	)	$	-

(a) Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt, Capital Lease Obligations And Preferred Stock

	March 31, 2012				December 31, 2011
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
Ameren:(a)(b)
Long-term debt and capital lease obligations (including current portion)	$	6,856	$	7,673	$	6,856	$	7,800
Preferred stock		142		93		142		92
Ameren Missouri:
Long-term debt and capital lease obligations (including current portion)	$	3,950	$	4,530	$	3,950	$	4,541
Preferred stock		80		55		80		55
Ameren Illinois:
Long-term debt (including current portion)	$	1,658	$	1,967	$	1,658	$	1,943
Preferred stock		62		38		62		37
Genco:
Long-term debt (including current portion)	$	824	$	693	$	824	$	839

(a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

(b) Preferred stock not subject to mandatory redemption of the Ameren subsidiaries along with the 20% noncontrolling interest of EEI is recorded in Noncontrolling Interests on the balance sheet.