Consolidated Statement Of Cash Flows (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Cash Flows From Operating Activities:
Net income	$ 181		$ 500
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on asset impairments and other charges	628		126
(Gain) loss on sales of properties	(11)		(12)
Net mark-to-market loss on derivatives	6		15
Depreciation and amortization	552		558
Amortization of nuclear fuel	63		51
Amortization of debt issuance costs and premium/discounts	17		17
Deferred income taxes and investment tax credits, net	40		302
Allowance for equity funds used during construction	(26)	[1]	(25)	[1]
Other	22		2
Changes in assets and liabilities:
Receivables	(19)		130
Materials and supplies	(44)		(34)
Accounts and wages payable	(157)		(192)
Taxes accrued	97		94
Assets, other	(29)		96
Liabilities, other	137		(2)
Pension and other postretirement benefits	19		(98)
Counterparty collateral, net	23		37
Premiums paid on long-term debt repurchases	(138)
Net cash provided by operating activities	1,361		1,565
Cash Flows From Investing Activities:
Capital expenditures	(905)		(758)
Nuclear fuel expenditures	(56)		(45)
Purchases of securities - nuclear decommissioning trust fund	(341)		(163)
Sales of securities - nuclear decommissioning trust fund	277		147
Proceeds from sales of properties	22		50
Other	(8)		18
Net cash used in investing activities	(1,011)		(751)
Cash Flows From Financing Activities:
Dividends on common stock	(284)		(279)
Dividends paid to noncontrolling interest holders	(5)		(5)
Short-term debt and credit facility repayments, net	(143)		(379)
Redemptions, repurchases, and maturities of long-term debt	(754)		(150)
Issuances:
Long-term debt	882
Common stock			49
Capital issuance costs	(7)
Generator advances received for construction	4
Repayments of generator advances received for construction			(73)
Net cash used in financing activities	(307)		(837)
Net change in cash and cash equivalents	43		(23)
Cash and cash equivalents at beginning of year	255		545
Cash and cash equivalents at end of period	298		522
Noncash financing activity - dividends on common stock	(7)
Union Electric Company [Member]
Cash Flows From Operating Activities:
Net income	403		304
Adjustments to reconcile net income to net cash provided by operating activities:
Loss from regulatory disallowance			89
Depreciation and amortization	303		278
Amortization of nuclear fuel	63		51
Amortization of debt issuance costs and premium/discounts	5		5
Deferred income taxes and investment tax credits, net	217		203
Allowance for equity funds used during construction	(23)		(22)
Other	7		(5)
Changes in assets and liabilities:
Receivables	(62)		(46)
Materials and supplies	(53)		2
Accounts and wages payable	(168)		(143)
Taxes accrued	59		51
Assets, other	(29)		82
Liabilities, other	71		3
Pension and other postretirement benefits	17		2
Premiums paid on long-term debt repurchases	(62)
Net cash provided by operating activities	748		854
Cash Flows From Investing Activities:
Capital expenditures	(445)		(402)
Nuclear fuel expenditures	(56)		(45)
Purchases of securities - nuclear decommissioning trust fund	(341)		(163)
Sales of securities - nuclear decommissioning trust fund	277		147
Other	(5)		4
Net cash used in investing activities	(570)		(459)
Cash Flows From Financing Activities:
Dividends on common stock	(300)		(219)
Dividends on preferred stock	(3)		(3)
Redemptions, repurchases, and maturities of long-term debt	(422)
Issuances:
Long-term debt	482
Capital issuance costs	(4)
Repayments of generator advances received for construction			(19)
Net cash used in financing activities	(247)		(241)
Net change in cash and cash equivalents	(69)		154
Cash and cash equivalents at beginning of year	201		202
Cash and cash equivalents at end of period	132		356
Ameren Illinois Company [Member]
Cash Flows From Operating Activities:
Net income	132		170
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	159		154
Amortization of debt issuance costs and premium/discounts	7		6
Deferred income taxes and investment tax credits, net	127		137
Other	(8)		(7)
Changes in assets and liabilities:
Receivables	58		169
Materials and supplies	(6)		(41)
Accounts and wages payable	(4)		(58)
Taxes accrued	(3)		(14)
Assets, other	(2)		3
Liabilities, other	42		(13)
Pension and other postretirement benefits	(8)		(98)
Counterparty collateral, net	23		29
Premiums paid on long-term debt repurchases	(76)
Net cash provided by operating activities	441		437
Cash Flows From Investing Activities:
Capital expenditures	(309)		(261)
Returns of advances from ATXI for construction			49
Other	5		6
Net cash used in investing activities	(304)		(206)
Cash Flows From Financing Activities:
Dividends on common stock	(132)		(238)
Dividends on preferred stock	(2)		(2)
Redemptions, repurchases, and maturities of long-term debt	(332)		(150)
Issuances:
Long-term debt	400
Capital issuance costs	(3)
Generator advances received for construction	4
Repayments of generator advances received for construction			(53)
Capital contribution from parent			6
Net cash used in financing activities	(65)		(437)
Net change in cash and cash equivalents	72		(206)
Cash and cash equivalents at beginning of year	21		322
Cash and cash equivalents at end of period	93		116
Ameren Energy Generating Company [Member]
Cash Flows From Operating Activities:
Net income	2		31
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on asset impairments and other charges			36
(Gain) loss on sales of properties	1		(12)
Net mark-to-market loss on derivatives	10		5
Depreciation and amortization	61		75
Amortization of debt issuance costs and premium/discounts	2		2
Deferred income taxes and investment tax credits, net	12		45
Other	8
Changes in assets and liabilities:
Receivables	22		22
Materials and supplies	7		6
Accounts and wages payable			(16)
Taxes accrued	(7)		(6)
Assets, other	(3)		3
Liabilities, other	(7)		(9)
Pension and other postretirement benefits	3		(3)
Net cash provided by operating activities	111		179
Cash Flows From Investing Activities:
Capital expenditures	(140)		(112)
Proceeds from sales of properties	4		49
Money pool advances, net	42		(38)
Net cash used in investing activities	(94)		(101)
Cash Flows From Financing Activities:
Credit facility repayments, net			(100)
Issuances:
Capital contribution from parent			24
Net cash used in financing activities			(76)
Net change in cash and cash equivalents	17		2
Cash and cash equivalents at beginning of year	8		6
Cash and cash equivalents at end of period	$ 25		$ 8

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations