Consolidated Statement Of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Cash Flows From Operating Activities:
Net income (loss)	$ (403)		$ 74
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Loss on asset impairment	628
Depreciation and amortization	188		187
Amortization of nuclear fuel	21		17
Amortization of debt issuance costs and premium/discounts	5		5
Deferred income taxes and investment tax credits, net	(142)		(16)
Allowance for equity funds used during construction	(9)	[1]	(6)	[1]
Net mark-to-market (gain) loss on derivatives	(3)		(16)
Other	(5)
Changes in assets and liabilities:
Receivables	109		94
Materials and supplies	80		135
Accounts and wages payable	(220)		(213)
Taxes accrued	35		71
Assets, other	14		50
Liabilities, other	64		80
Pension and other postretirement benefits	41		28
Counterparty collateral, net	(11)		70
Net cash provided by operating activities	392		560
Cash Flows From Investing Activities:
Capital expenditures	(282)		(231)
Nuclear fuel expenditures	(38)		(22)
Purchases of securities - nuclear decommissioning trust fund	(109)		(91)
Sales of securities - nuclear decommissioning trust fund	88		87
Proceeds from sale of property	16
Other	(1)		1
Net cash used in investing activities	(326)		(256)
Cash Flows From Financing Activities:
Dividends on common stock	(90)		(93)
Dividends paid to noncontrolling interest holders	(2)		(2)
Short-term debt and credit facility borrowings, net	(22)		(125)
Generator advances received for construction	1
Repayments of generator advances received for construction			(73)
Issuances of common stock			17
Net cash Provided by (used in) financing activities	(113)		(276)
Net change in cash and cash equivalents	(47)		28
Cash and cash equivalents at beginning of year	255		545
Cash and cash equivalents at end of period	208		573
Noncash financing activity - dividends on common stock	(7)
Union Electric Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	22		22
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	100		93
Amortization of nuclear fuel	21		17
Amortization of debt issuance costs and premium/discounts	2		2
Deferred income taxes and investment tax credits, net	2		9
Allowance for equity funds used during construction	(8)		(6)
Net mark-to-market (gain) loss on derivatives			1
Changes in assets and liabilities:
Receivables	61		16
Materials and supplies	(26)		14
Accounts and wages payable	(136)		(144)
Taxes accrued	39		(1)
Assets, other	13		29
Liabilities, other	14		14
Pension and other postretirement benefits	17		14
Net cash provided by operating activities	121		80
Cash Flows From Investing Activities:
Capital expenditures	(157)		(129)
Nuclear fuel expenditures	(38)		(22)
Purchases of securities - nuclear decommissioning trust fund	(109)		(91)
Sales of securities - nuclear decommissioning trust fund	88		87
Other	(2)		(1)
Net cash used in investing activities	(218)		(156)
Cash Flows From Financing Activities:
Dividends on common stock	(100)		(68)
Dividends on preferred stock	(1)		(1)
Generator advances received for construction			(19)
Net cash Provided by (used in) financing activities	(101)		(88)
Net change in cash and cash equivalents	(198)		(164)
Cash and cash equivalents at beginning of year	201		202
Cash and cash equivalents at end of period	3		38
Ameren Illinois Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	28		34
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	52		50
Amortization of debt issuance costs and premium/discounts	2		2
Deferred income taxes and investment tax credits, net	55		(31)
Other	(2)		(1)
Changes in assets and liabilities:
Receivables	35		42
Materials and supplies	103		123
Accounts and wages payable	(16)		(47)
Taxes accrued			46
Assets, other	2		12
Liabilities, other	26		42
Pension and other postretirement benefits	15		11
Counterparty collateral, net	(11)		32
Net cash provided by operating activities	289		315
Cash Flows From Investing Activities:
Capital expenditures	(86)		(69)
Returns of advances from ATXI for construction			49
Other			1
Net cash used in investing activities	(86)		(19)
Cash Flows From Financing Activities:
Capital contribution from parent			6
Dividends on common stock	(37)		(62)
Dividends on preferred stock	(1)		(1)
Generator advances received for construction	1
Repayments of generator advances received for construction			(53)
Net cash Provided by (used in) financing activities	(37)		(110)
Net change in cash and cash equivalents	166		186
Cash and cash equivalents at beginning of year	21		322
Cash and cash equivalents at end of period	187		508
Noncash investing activity - asset transfer from ATXI			20
Ameren Energy Generating Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	(3)		22
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	23		25
Amortization of debt issuance costs and premium/discounts	1		1
Deferred income taxes and investment tax credits, net	(7)		13
Net mark-to-market (gain) loss on derivatives	(1)		(15)
Other	5
Changes in assets and liabilities:
Receivables	27		18
Materials and supplies	(1)		4
Accounts and wages payable	(9)		(16)
Taxes accrued	1		17
Assets, other	(4)		(3)
Liabilities, other	13		12
Pension and other postretirement benefits	1		(2)
Net cash provided by operating activities	46		76
Cash Flows From Investing Activities:
Capital expenditures	(33)		(35)
Money pool advances, net	(21)		(65)
Net cash used in investing activities	(54)		(100)
Cash Flows From Financing Activities:
Capital contribution from parent			24
Net cash Provided by (used in) financing activities			24
Net change in cash and cash equivalents	(8)
Cash and cash equivalents at beginning of year	8		6
Cash and cash equivalents at end of period			$ 6

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