Consolidated Statement Of Cash Flows (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Cash Flows From Operating Activities:
Net income (loss)	$ (974)		$ 526		$ 151
Adjustments to reconcile net income to net cash provided by operating activities:
Impairment and other charges	2,578	[1]	125	[1]	589	[1]
Gain on sales of properties	(11)		(15)		(10)
Net mark-to-market (gain) loss on derivatives	22		11		(15)
Depreciation and amortization	735		747		746
Amortization of nuclear fuel	83		61		54
Amortization of debt issuance costs and premium/discounts	24		21		23
Deferred income taxes and investment tax credits, net	(714)		346		410
Allowance for equity funds used during construction	(36)	[2]	(34)	[2]	(52)	[2]
Other	25				21
Changes in assets and liabilities:
Receivables	33		231		(197)
Materials and supplies	5		(27)		73
Accounts and wages payable	(29)		(36)		20
Taxes accrued	3		(3)		10
Assets, other	(10)		76		(47)
Liabilities, other	71		(75)		71
Pension and other postretirement benefits	(23)		(102)		(5)
Counterparty collateral, net	46		27		(73)
Premiums paid on long-term debt repurchases	(138)
Taum Sauk insurance recoveries, net of cost			(1)		54
Net cash provided by operating activities	1,690		1,878		1,823
Cash Flows From Investing Activities:
Capital expenditures	(1,240)		(1,030)		(1,042)
Nuclear fuel expenditures	(91)		(62)		(68)
Purchases of securities - nuclear decommissioning trust fund	(403)		(220)		(271)
Sales and maturities of securities - nuclear decommissioning trust fund	384		199		256
Proceeds from sales of properties	22		53		27
Tax grants received related to renewable energy properties	18
Other			12		2
Net cash used in investing activities	(1,310)		(1,048)		(1,096)
Cash Flows From Financing Activities:
Dividends on common stock	(382)		(375)		(368)
Dividends paid to noncontrolling interest holders	(6)		(6)		(8)
Short-term debt and credit facility repayments, net	(148)		(581)		(121)
Redemptions, repurchases, and maturities:
Long-term debt	(760)		(155)		(310)
Preferred stock					(52)
Issuances:
Long-term debt	882
Common stock			65		80
Capital issuance costs	(16)				(15)
Generator advances received for construction	4		5		29
Repayments of generator advances received for construction			(73)		(39)
Net cash provided by (used in) financing activities	(426)		(1,120)		(804)
Net change in cash and cash equivalents	(46)		(290)		(77)
Cash and cash equivalents at beginning of year	255		545		622
Cash and cash equivalents at end of year	209		255		545
Noncash financing activity – dividends on common stock	(7)
Cash Paid (Refunded) During the Year:
Interest net of capitalized	433		453		494
Income taxes, net	1		(61)		(92)
Union Electric Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	419		290		369
Adjustments to reconcile net income to net cash provided by operating activities:
Loss from regulatory disallowance			89
Gain on sales of properties			(3)		(5)
Net mark-to-market (gain) loss on derivatives			1		(1)
Depreciation and amortization	407		377		355
Amortization of nuclear fuel	83		61		54
Amortization of debt issuance costs and premium/discounts	6		6		4
Deferred income taxes and investment tax credits, net	287		155		292
Allowance for equity funds used during construction	(31)		(30)		(50)
Other	8		(6)		10
Changes in assets and liabilities:
Receivables	27		66		(122)
Materials and supplies	(48)		(7)		7
Accounts and wages payable	(27)		13		(24)
Taxes accrued	(46)		(6)		55
Assets, other	(35)		79		(101)
Liabilities, other	14		(30)		75
Pension and other postretirement benefits	2		2		(3)
Premiums paid on long-term debt repurchases	(62)
Taum Sauk insurance recoveries, net of cost			(1)		54
Net cash provided by operating activities	1,004		1,056		969
Cash Flows From Investing Activities:
Capital expenditures	(595)		(550)		(624)
Nuclear fuel expenditures	(91)		(62)		(68)
Purchases of securities - nuclear decommissioning trust fund	(403)		(220)		(271)
Sales and maturities of securities - nuclear decommissioning trust fund	384		199		256
Proceeds from sales of properties			27
Money pool advances, net	(24)
Tax grants received related to renewable energy properties	18
Other	8		6		7
Net cash used in investing activities	(703)		(627)		(700)
Cash Flows From Financing Activities:
Dividends on common stock	(400)		(403)		(235)
Dividends on preferred stock	(3)		(3)		(5)
Redemptions, repurchases, and maturities:
Long-term debt	(427)		(5)		(70)
Preferred stock					(33)
Issuances:
Long-term debt	482
Capital issuance costs	(7)				(4)
Capital contribution from parent	1
Generator advances received for construction					13
Repayments of generator advances received for construction			(19)
Net cash provided by (used in) financing activities	(354)		(430)		(334)
Net change in cash and cash equivalents	(53)		(1)		(65)
Cash and cash equivalents at beginning of year	201		202		267
Cash and cash equivalents at end of year	148		201		202
Cash Paid (Refunded) During the Year:
Interest net of capitalized	220		210		213
Income taxes, net	(3)		9		(106)
Ameren Illinois Company [Member]
Cash Flows From Operating Activities:
Net income (loss)	144		196		252
Income from discontinued operations, net of tax			0		(40)
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	214		206		201
Amortization of debt issuance costs and premium/discounts	11		8		10
Deferred income taxes and investment tax credits, net	104		155		210
Allowance for equity funds used during construction	(2)		(2)		(4)
Other	(11)		(14)		(3)
Changes in assets and liabilities:
Receivables	23		146		(84)
Materials and supplies	20		(21)		9
Accounts and wages payable	(21)		(46)		(44)
Taxes accrued	3		(12)		11
Assets, other	22		(3)		32
Liabilities, other	72		(30)		33
Pension and other postretirement benefits	(26)		(101)		(7)
Counterparty collateral, net	40		20		(100)
Premiums paid on long-term debt repurchases	(76)
Operating cash flows provided by discontinued operations					113
Net cash provided by operating activities	519		504		593
Cash Flows From Investing Activities:
Capital expenditures	(442)		(351)		(281)
Returns from (advances to) ATXI for construction	0		49		(10)
Proceeds from intercompany note receivable					45
Other	5		6		5
Capital expenditures of discontinued operations					(6)
Net cash used in investing activities	(437)		(296)		(247)
Cash Flows From Financing Activities:
Dividends on common stock	(189)		(327)		(133)
Dividends on preferred stock	(3)		(3)		(4)
Money pool borrowings, net	24
Short-term debt and credit facility repayments, net
Redemptions, repurchases, and maturities:
Long-term debt	(333)		(150)		(40)
Preferred stock					(19)
Issuances:
Long-term debt	400
Capital issuance costs	(6)				(4)
Capital contribution from parent			19
Generator advances received for construction	4		5		16
Repayments of generator advances received for construction			(53)		(39)
Net financing activities provided by (used in) discontinued operations					(107)
Net cash provided by (used in) financing activities	(103)		(509)		(330)
Net change in cash and cash equivalents	(21)		(301)		16
Cash and cash equivalents at beginning of year	21		322		306
Cash and cash equivalents at end of year	0		21		322
Cash Paid (Refunded) During the Year:
Interest net of capitalized	125		137		160
Income taxes, net	(22)		(14)		(39)
Noncash investing activity - asset transfer from ATXI					7
Noncash financing activity - capital contribution from parent					$ 6

[1] Includes amounts for registrant and nonregistrant subsidiaries.

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