Retirement Benefits (Tables)

12 Months Ended

Dec. 31, 2011

Summary Of Benefit Liability Recorded

Ameren(a)	$	1,350
Ameren Missouri		494
Ameren Illinois		496
Genco		141

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

Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI

	2011						2010
	Pension Benefits(a)			Postretirement Benefits(a)			Pension Benefits(a)			Postretirement Benefits(a)
Accumulated benefit obligation at end of year	$	3,645		$	(b	)	$	3,246		$	(b	)
Change in benefit obligation:
Net benefit obligation at beginning of year	$	3,451		$	1,120		$	3,255		$	1,143
Service cost		75			22			68			20
Interest cost		180			58			185			62
Plan amendments(c)(d)		(16	)		-			(40	)		-
Participant contributions		-			18			-			17
Actuarial (gain) loss		348			96			165			(53	)
Benefits paid		(173	)		(66	)		(182	)		(74	)
Early retiree reinsurance program receipt		(b	)		3			(b	)		-
Federal subsidy on benefits paid		(b	)		6			(b	)		5
Net benefit obligation at end of year		3,865			1,257			3,451			1,120
Change in plan assets:
Fair value of plan assets at beginning of year		2,722			797			2,495			732
Actual return on plan assets		224			9			328			81
Employer contributions		103			129			81			36
Federal subsidy on benefits paid		(b	)		6			(b	)		5
Early retiree reinsurance program receipt		(b	)		3			(b	)		-
Participant contributions		-			18			-			17
Benefits paid		(173	)		(66	)		(182	)		(74	)
Fair value of plan assets at end of year		2,876			896			2,722			797
Funded status – deficiency		989			361			729			323
Accrued benefit cost at December 31	$	989		$	361		$	729		$	323
Amounts recognized in the balance sheet consist of:
Current liability	$	3		$	3		$	4		$	3
Noncurrent liability		986			358			725			320
Total	$	989		$	361		$	729		$	323
Amounts recognized in regulatory assets consist of:
Net actuarial loss	$	734		$	177		$	507		$	86
Prior service cost (credit)		(7	)		(28	)		(11	)		(32	)
Transition obligation		-			2			-			5
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss		79			43			24			13
Prior service cost (credit)		(15	)		(7	)		4			(10	)
Total	$	791		$	187		$	524		$	62

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

(b) Not applicable.

(c) In 2011, Ameren's pension plan was amended to adjust the calculation of the future benefit obligation of approximately 430 labor union-represented employees from a traditional, final pay formula to a cash balance formula.

(d) In 2010, Ameren's pension plan was amended to adjust the calculation of the future benefit obligation of approximately 700 management employees from a traditional, final pay formula to a cash balance formula.

Assumptions Used To Determine Benefit Obligations

	Pension Benefits						Postretirement Benefits
	2011			2010			2011			2010
Discount rate at measurement date		4.50	%		5.25	%		4.50	%		5.25	%
Increase in future compensation		3.50			3.50			3.50			3.50
Medical cost trend rate (initial)		-			-			5.50			6.00
Medical cost trend rate (ultimate)		-			-			5.00			5.00
Years to ultimate rate		-			-			10 year			2 years

Schedule Of Cash Contributions Made To Benefit Plans

	Pension Benefits						Postretirement Benefits
	2011		2010		2009		2011		2010		2009
Ameren(a)	$	103	$	81	$	99	$	129	$	36	$	49
AMO		43		36		42		9		11		13
AIC		28		23		25		118		20		28
Genco		12		4		10		-		-		-

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

Target Allocation Of The Plans' Asset Categories

				Percentage of Plan Assets at December  31,
Asset Category	Target Allocation 2012			2011			2010
Pension Plan:
Cash and cash equivalents		0  - 5	%		2	%		1	%
Equity securities:
U.S. large capitalization		29 - 39			33			31
U.S. small and mid-capitalization		2 - 12			7			11
International and emerging markets		9 - 19			11			15
Total equity		50 - 60			51			57
Debt securities		35 - 45			42			37
Real estate		0 - 9			4			4
Private equity		0 - 4			1			1
Total					100	%		100	%
Postretirement Plans:
Cash and cash equivalents		0 - 10	%		4	%		4	%
Equity securities:
U.S. large capitalization		33 - 43			38			39
U.S. small and mid-capitalization		3 - 13			8			10
International		10 - 20			13			14
Total equity		55 - 65			59			63
Debt securities		30 - 40			37			33
Total					100	%		100	%

Changes In The Fair Value Of Plan Assets Classified As Level 3

	Beginning Balance at January 1,		Actual Return on Plan Assets Related to Assets Still Held at the Reporting Date			Actual Return on Plan Assets Related to Assets Sold During the Period		Purchases, Sales, and Settlements, net			Net Transfers into (out of) of Level 3		Ending Balance at December 31,
2011:
Real estate	$	98	$	10		$	-	$	-		$	-	$	108
Private equity		28		(10	)		11		(6	)		-		23
2010:
Other debt securities	$	1	$	-		$	-	$	(1	)	$	-	$	-
Real estate		90		7			-		1			-		98
Private equity		33		(5	)		7		(7	)		-		28

Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost

	Pension Benefits			Postretirement Benefits
	Ameren(a)			Ameren(a)
Regulatory assets:
Transition obligation	$	-		$	2
Prior service cost (credit)		(1	)		(4	)
Net actuarial loss		87			23
Accumulated OCI:
Transition obligation		-			-
Prior service cost (credit)		(1	)		(1	)
Net actuarial loss		6			3
Total	$	91		$	23

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

Components Of Net Periodic Benefit Cost

	Pension Benefits			Postretirement Benefits
	Ameren(a)			Ameren(a)
2011:
Service cost	$	75		$	22
Interest cost		180			58
Expected return on plan assets		(216	)		(54	)
Amortization of:
Transition obligation		-			2
Prior service cost		(1	)		(8	)
Actuarial loss		42			5
Net periodic benefit cost	$	80		$	25
2010:
Service cost	$	68		$	20
Interest cost		185			62
Expected return on plan assets		(212	)		(56	)
Amortization of:
Transition obligation		-			2
Prior service cost		6			(8	)
Actuarial loss		18			1
Net periodic benefit cost	$	65		$	21
2009:
Service cost	$	68		$	19
Interest cost		186			66
Expected return on plan assets		(206	)		(54	)
Amortization of:
Transition obligation		-			2
Prior service cost		9			(8	)
Actuarial loss		24			9
Net periodic benefit cost	$	81		$	34

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

Summary Of Benefit Plan Costs Incurred

	Pension Costs						Postretirement Costs
	2011		2010		2009		2011		2010		2009
Ameren(a)	$	80	$	65	$	81	$	25	$	21	$	34
Ameren Missouri		51		42		50		11		11		15
Ameren Illinois		16		10		14		11		7		16
Genco		8		9		11		3		2		3

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

Schedule Of Expected Payments From Qualified Trust And Company Funds

	Pension Benefits				Postretirement Benefits
	Paid from Qualified Trust		Paid from Company Funds		Paid from Qualified Trust		Paid from Company Funds		Federal Subsidy
2012		223		3		68		3		5
2013		225		3		71		3		5
2014		230		3		74		3		5
2015		231		3		77		3		6
2016		232		3		80		3		6
2017 - 2021		1,167		12		443		14		32

Assumptions Used To Determine Net Periodic Benefit Cost

	Pension Benefits									Postretirement Benefits
	2011			2010			2009			2011			2010			2009
Discount rate at measurement date		5.25	%		5.75	%		5.75	%		5.25	%		5.75	%		5.75	%
Expected return on plan assets		8.00			8.00			8.00			7.75			8.00			8.00
Increase in future compensation		3.50			3.50			4.00			3.50			3.50			4.00
Medical cost trend rate (initial)		-			-			-			6.00			6.50			7.00
Medical cost trend rate (ultimate)		-			-			-			5.00			5.00			5.00
Years to ultimate rate		-			-			-			2 years			3 years			4 years

Schedule Of Potential Changes In Key Assumptions

	Pension Benefits					Postretirement Benefits
	Service Cost and Interest Cost			Projected Benefit Obligation		Service Cost and Interest Cost			Postretirement Benefit Obligation
0.25% decrease in discount rate	$	(2	)	$	110	$	-		$	38
0.25% increase in salary scale		2			14		-			-
1.00% increase in annual medical trend		-			-		3			42
1.00% decrease in annual medical trend		-			-		(3	)		(41	)

Schedule Of Matching Contributions

	2011		2010		2009
Ameren(a)	$	28	$	27	$	24
Ameren Missouri		16		16		14
Ameren Illinois		8		8		7
Genco		2		1		2

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

Pension Benefits [Member]

Schedule of defined benefit plan fair value of plan assets by measurement

	Quoted Prices in Active Markets for Identified Assets (Level 1)			Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	-		$	31	$	-	$	31
Equity securities:
U.S. large capitalization		72			922		-		994
U.S. small and mid-capitalization		202			11		-		213
International and emerging markets		115			213		-		328
Debt securities:
Corporate bonds		-			720		-		720
Municipal bonds		-			176		-		176
U.S. treasury and agency securities		-			230		-		230
Other		-			121		-		121
Real estate		-			-		108		108
Private equity		-			-		23		23
Derivative assets		1			-		-		1
Derivative liabilities		(1	)		-		-		(1	)
Total	$	389		$	2,424	$	131	$	2,944
Less: Medical benefit assets at December 31(a)									(91	)
Plus: Net receivables at December 31(b)									23
Fair value of pension plans assets at year end								$	2,876

(a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code (401(h) accounts) to fund a portion of the postretirement obligation.

(b) Receivables related to pending security sales, offset by payables related to pending security purchases.

The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2010:

	Quoted Prices in Active Markets for Identified Assets (Level 1)			Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	-		$	20	$	-	$	20
Equity securities:
U.S. large capitalization		70			812		-		882
U.S. small and mid-capitalization		299			10		-		309
International and emerging markets		129			284		-		413
Debt securities:
Corporate bonds		-			646		-		646
Municipal bonds		-			129		-		129
U.S. treasury and agency securities		-			154		-		154
Other		-			100		-		100
Real estate		-			-		98		98
Private equity		-			-		28		28
Derivative assets		1			-		-		1
Derivative liabilities		(1	)		-		-		(1	)
Total	$	498		$	2,155	$	126	$	2,779
Less: Medical benefit assets at December 31(a)									(85	)
Plus: Net receivables at December 31(b)									28
Fair value of pension plans assets at year end								$	2,722

(a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code (401(h) accounts) to fund a portion of the postretirement obligation.

(b) Receivables related to pending security sales, offset by payables related to pending security purchases.

Target Allocation Of The Plans' Asset Categories

	Quoted Prices in Active Markets for Identified Assets (Level 1)			Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	-		$	31	$	-	$	31
Equity securities:
U.S. large capitalization		72			922		-		994
U.S. small and mid-capitalization		202			11		-		213
International and emerging markets		115			213		-		328
Debt securities:
Corporate bonds		-			720		-		720
Municipal bonds		-			176		-		176
U.S. treasury and agency securities		-			230		-		230
Other		-			121		-		121
Real estate		-			-		108		108
Private equity		-			-		23		23
Derivative assets		1			-		-		1
Derivative liabilities		(1	)		-		-		(1	)
Total	$	389		$	2,424	$	131	$	2,944
Less: Medical benefit assets at December 31(a)									(91	)
Plus: Net receivables at December 31(b)									23
Fair value of pension plans assets at year end								$	2,876

(a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code (401(h) accounts) to fund a portion of the postretirement obligation.

(b) Receivables related to pending security sales, offset by payables related to pending security purchases.

The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2010:

	Quoted Prices in Active Markets for Identified Assets (Level 1)			Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	-		$	20	$	-	$	20
Equity securities:
U.S. large capitalization		70			812		-		882
U.S. small and mid-capitalization		299			10		-		309
International and emerging markets		129			284		-		413
Debt securities:
Corporate bonds		-			646		-		646
Municipal bonds		-			129		-		129
U.S. treasury and agency securities		-			154		-		154
Other		-			100		-		100
Real estate		-			-		98		98
Private equity		-			-		28		28
Derivative assets		1			-		-		1
Derivative liabilities		(1	)		-		-		(1	)
Total	$	498		$	2,155	$	126	$	2,779
Less: Medical benefit assets at December 31(a)									(85	)
Plus: Net receivables at December 31(b)									28
Fair value of pension plans assets at year end								$	2,722

(a) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code (401(h) accounts) to fund a portion of the postretirement obligation.

(b) Receivables related to pending security sales, offset by payables related to pending security purchases.

Postretirement Benefits [Member]

Schedule of defined benefit plan fair value of plan assets by measurement

	Quoted Prices in Active Markets for Identified Assets (Level 1)			Significant Other Observable Inputs (Level 2)			Significant Other Unobservable Inputs (Level 3)			Total
Cash and cash equivalents		$	1		$	66		$	-		$	67
Equity securities:
U.S. large capitalization			235			78			-			313
U.S. small and mid-capitalization			57			-			-			57
International			44			56			-			100
Debt securities:
Corporate bonds			-			61			-			61
Municipal bonds			-			86			-			86
U.S. treasury and agency securities			-			82			-			82
Asset-backed securities			-			23			-			23
Other			-			49			-			49
Total		$	337		$	501		$	-		$	838
Plus: Medical benefit assets at December 31(a)												91
Less: Net payables at December 31(b)												(33	)
Fair value of postretirement benefit plans assets at year end											$	896

(a) Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.

(b) Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.

The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2010:

	Quoted Prices in Active Markets for Identified Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	-	$	35	$	-	$	35
Equity securities:
U.S. large capitalization		215		72		-		287
U.S. small and mid-capitalization		66		-		-		66
International		43		51		-		94
Debt securities:
Corporate bonds		-		59		-		59
Municipal bonds		-		58		-		58
U.S. treasury and agency securities		-		59		-		59
Asset-backed securities		-		31		-		31
Other		-		29		-		29
Total	$	324	$	394	$	-	$	718
Plus: Medical benefit assets at December 31(a)								85
Less: Net payables at December 31(b)								(6	)
Fair value of postretirement benefit plans assets at year end							$	797

(a) Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.

(b) Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.

Target Allocation Of The Plans' Asset Categories

	Quoted Prices  in Active Markets for Identified Assets (Level 1)		Significant  Other Observable Inputs (Level 2)		Significant  Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	1	$	66	$	—	$	67
Equity securities:
U.S. large capitalization		235		78		—		313
U.S. small and mid-capitalization		57		—		—		57
International		44		56		—		100
Debt securities:
Corporate bonds		—		61		—		61
Municipal bonds		—		86		—		86
U.S. treasury and agency securities		—		82		—		82
Asset-backed securities		—		23		—		23
Other		—		49		—		49
Total	$	337	$	501	$	—	$	838	(a)(b)

(a) Excludes $91 million of medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.

(b) Excludes $33 million of payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.

The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2010:

	Quoted Prices  in Active Markets for Identified Assets (Level 1)		Significant  Other Observable Inputs (Level 2)		Significant  Other Unobservable Inputs (Level 3)		Total
Cash and cash equivalents	$	—	$	35	$	—	$	35
Equity securities:
U.S. large capitalization		215		72		—		287
U.S. small and mid-capitalization		66		—		—		66
International		43		51		—		94
Debt securities:
Corporate bonds		—		59		—		59
Municipal bonds		—		58		—		58
U.S. treasury and agency securities		—		59		—		59
Asset-backed securities		—		31		—		31
Other		—		29		—		29
Total	$	324	$	394	$	—	$	718	(a)(b)

(a) Excludes $85 million of medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.

(b) Excludes $6 million of payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.