Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,509	[1]	$ 2,001	[1]	$ 1,660	[1]	$ 1,658	[1]	$ 1,578	[1]	$ 2,268	[1]	$ 1,781	[1]	$ 1,904	[1]	$ 6,828		$ 7,531		$ 7,638
Impairment and other charges																	2,578	[2]	125	[2]	589	[2]
Operating expenses																	8,068		6,290		6,722
Operating Income (Loss)	(1,816)	[1],[3]	635	[1],[3]	363	[1],[3]	(422)	[1],[3]	148	[1],[3]	550	[1],[3]	316	[1],[3]	227	[1],[3]	(1,240)		1,241		916
Interest income from affiliates																	5	[4],[5]	4	[4]	5	[4]
Miscellaneous expense																	71	[4]	69	[4]	90	[4]
Interest charges																	(448)		(451)		(497)
Income tax (benefit)																	(680)	[4]	310	[4]	325	[4]
Net income (loss)	(1,156)	[1]	374	[1]	211	[1]	(403)	[1]	25	[1]	285	[1]	138	[1]	71	[1]	(974)	[6]	519	[6]	139	[6]
Other Comprehensive Income (Loss), Net of Taxes:
Unrealized net gain (loss) on derivative hedging instruments, net of income taxes (benefit) of $12, $1, and $(1), respectively																	22		3		(2)
Reclassification adjustments for derivative (gains) losses included in net income, net of income taxes (benefit) of $1, $(3), and $5, respectively																	(4)		4		(8)
Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $22, $(32), and $6, respectively																	32		(46)		4
Total other comprehensive income (loss), net of taxes																	50		(39)		(6)
Comprehensive Income (Loss)																	(932)		486		135
Other Comprehensive Income (Loss), Taxes:
Unrealized net gain (loss) on derivative hedging instruments, tax (benefit)																	12		1		(1)
Reclassification adjustments for derivative (gain) included in net income, tax																	1		(3)		5
Pension and other postretirement activity, tax (benefit)																	22		(32)		6
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues
Impairment and other charges																					372
Operating expenses																	22		15		24
Operating Income (Loss)																	(22)		(15)		(396)
Equity in earnings (loss) of subsidiaries																	(954)		527		535
Interest income from affiliates																	40		44		28
Miscellaneous expense																	4		4		3
Interest charges																	(39)		(41)		(56)
Income tax (benefit)																	(5)		(8)		(31)
Net income (loss)																	(974)		519		139
Other Comprehensive Income (Loss), Net of Taxes:
Unrealized net gain (loss) on derivative hedging instruments, net of income taxes (benefit) of $12, $1, and $(1), respectively																	22		3		(2)
Reclassification adjustments for derivative (gains) losses included in net income, net of income taxes (benefit) of $1, $(3), and $5, respectively																	(4)		4		(8)
Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $22, $(32), and $6, respectively																	32		(46)		4
Total other comprehensive income (loss), net of taxes																	50		(39)		(6)
Comprehensive Income (Loss)																	(924)		480		133
Other Comprehensive Income (Loss), Taxes:
Unrealized net gain (loss) on derivative hedging instruments, tax (benefit)																	12		1		(1)
Reclassification adjustments for derivative (gain) included in net income, tax																	1		(3)		5
Pension and other postretirement activity, tax (benefit)																	$ 22		$ (32)		$ 6

[1] The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is due to the effects of rounding and changes in the number of weighted-average shares outstanding each period.

[2] Includes amounts for registrant and nonregistrant subsidiaries.

[3] Includes pretax "Impairment and other charges" of $2,578 million and $125 million recorded at Ameren during the years ended December 31, 2012, and 2011, respectively. See Note 17 - Impairment and Other Charges under Part II, Item 8, for additional information.

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

[5] Includes interest income relating to a 2012 refund of charges included in an expired power purchase agreement with Entergy. See Note 2 - Rate and Regulatory Matters for additional information.

[6] Represents net income (loss) available to common stockholders.