Consolidated Statement Of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Cash Flows From Operating Activities:
Net income (loss)	$ (143)		$ (403)
Loss from discontinued operations, net of taxes	199		442
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	166		156
Amortization of nuclear fuel	20		21
Amortization of debt issuance costs and premium/discounts	6		4
Deferred income taxes and investment tax credits, net	40		30
Allowance for equity funds used during construction	(8)	[1]	(9)	[1]
Stock-based compensation costs	9		8
Other	(3)		(5)
Changes in assets and liabilities:
Receivables	(95)		93
Materials and supplies	127		79
Accounts and wages payable	(127)		(208)
Taxes accrued	41		22
Assets, other	52		19
Liabilities, other	29		23
Pension and other postretirement benefits	3		41
Counterparty collateral, net	26		(9)
Net cash provided by operating activities - continuing operations	342		304
Net cash provided by operating activities - discontinued operations	37		79
Net cash provided by operating activities	379		383
Cash Flows From Investing Activities:
Capital expenditures	(275)		(245)
Nuclear fuel expenditures	(11)		(38)
Purchases of securities - nuclear decommissioning trust fund	(35)		(109)
Sales and maturities of securities - nuclear decommissioning trust fund	32		102
Other	(2)		(2)
Net cash used in investing activities - continuing operations	(291)		(292)
Net cash used in investing activities - discontinued operations	(12)		(19)
Net cash used in investing activities	(303)		(311)
Cash Flows From Financing Activities:
Dividends on common stock	(97)		(90)
Dividends paid to noncontrolling interest holders	(2)		(2)
Short-term debt, net	0		(22)
Advances received for construction	0		1
Net cash used in financing activities - continuing operations	(99)		(113)
Net cash used in financing activities - discontinued operations	0		0
Net cash used in financing activities	(99)		(113)
Net change in cash and cash equivalents	(23)		(41)
Cash and cash equivalents at beginning of year	184		248
Cash and cash equivalents at end of period	161		207
Noncash financing activity - dividends on common stock			(7)
Union Electric Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	41		22
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	104		100
Amortization of nuclear fuel	20		21
Amortization of debt issuance costs and premium/discounts	2		2
Deferred income taxes and investment tax credits, net	(8)		2
Allowance for equity funds used during construction	(7)		(8)
Changes in assets and liabilities:
Receivables	(50)		61
Materials and supplies	22		(26)
Accounts and wages payable	(139)		(136)
Taxes accrued	38		39
Assets, other	38		13
Liabilities, other	5		0
Pension and other postretirement benefits	2		17
Net cash provided by operating activities	68		107
Cash Flows From Investing Activities:
Capital expenditures	(137)		(157)
Nuclear fuel expenditures	(11)		(38)
Purchases of securities - nuclear decommissioning trust fund	(35)		(109)
Sales and maturities of securities - nuclear decommissioning trust fund	32		102
Money pool advances, net	24		0
Other	(2)		(2)
Net cash used in investing activities	(129)		(204)
Cash Flows From Financing Activities:
Dividends on common stock	(90)		(100)
Dividends on preferred stock	(1)		(1)
Money pool borrowings, net	5		0
Net cash used in financing activities	(86)		(101)
Net change in cash and cash equivalents	(147)		(198)
Cash and cash equivalents at beginning of year	148		201
Cash and cash equivalents at end of period	1		3
Ameren Illinois Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	32		28
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	60		52
Amortization of debt issuance costs and premium/discounts	3		2
Deferred income taxes and investment tax credits, net	50		55
Other	(1)		(2)
Changes in assets and liabilities:
Receivables	(58)		35
Materials and supplies	105		103
Accounts and wages payable	9		(16)
Taxes accrued	3		0
Assets, other	16		2
Liabilities, other	24		26
Pension and other postretirement benefits	1		15
Counterparty collateral, net	27		(11)
Net cash provided by operating activities	271		289
Cash Flows From Investing Activities:
Capital expenditures	(133)		(86)
Money pool advances, net	(5)		0
Net cash used in investing activities	(138)		(86)
Cash Flows From Financing Activities:
Dividends on common stock	(15)		(37)
Dividends on preferred stock	(1)		(1)
Money pool borrowings, net	(24)		0
Advances received for construction	0		1
Net cash used in financing activities	(40)		(37)
Net change in cash and cash equivalents	93		166
Cash and cash equivalents at beginning of year	0		21
Cash and cash equivalents at end of period	$ 93		$ 187

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