Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Details) (USD $) In Millions, unless otherwise specified			12 Months Ended					12 Months Ended																																																																																																												1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Power [Member]	Dec. 31, 2011 Interest Rate Contract [Member]	Dec. 31, 2012 Fuel Oils [Member]	Dec. 31, 2012 Natural Gas [Member]	Dec. 31, 2012 Uranium [Member]	Dec. 31, 2012 Interest Rate Swap [Member]	Dec. 31, 2011 Interest Rate Swap [Member]	Dec. 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2011 Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2011 Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2012 Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2011 Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2012 Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2011 Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2012 Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2011 Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2012 Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2011 Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2012 Ameren Missouri [Member]	Dec. 31, 2011 Ameren Missouri [Member]	Dec. 31, 2012 Ameren Missouri [Member] Power [Member]	Dec. 31, 2012 Ameren Missouri [Member] Fuel Oils [Member]	Dec. 31, 2012 Ameren Missouri [Member] Natural Gas [Member]	Dec. 31, 2012 Ameren Missouri [Member] Uranium [Member]	Dec. 31, 2012 Ameren Missouri [Member] Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2011 Ameren Missouri [Member] Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2012 Ameren Missouri [Member] Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2011 Ameren Missouri [Member] Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2012 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2011 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2012 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2011 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2012 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2011 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2012 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2011 Ameren Missouri [Member] Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2012 Ameren Illinois [Member]	Dec. 31, 2011 Ameren Illinois [Member]	Dec. 31, 2012 Ameren Illinois [Member] Power [Member]	Dec. 31, 2012 Ameren Illinois [Member] Natural Gas [Member]	Dec. 31, 2012 Ameren Illinois [Member] Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2011 Ameren Illinois [Member] Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2012 Ameren Illinois [Member] Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2011 Ameren Illinois [Member] Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2012 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2011 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2012 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2011 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2012 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2011 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2012 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2011 Ameren Illinois [Member] Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2012 Other [Member] Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2011 Other [Member] Accumulated Other Comprehensive Income (Loss) [Member] Power [Member]		Dec. 31, 2012 Other [Member] Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2011 Other [Member] Accumulated Other Comprehensive Income (Loss) [Member] Interest Rate Contract [Member]		Dec. 31, 2012 Other [Member] Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2011 Other [Member] Regulatory Liabilities Or Assets [Member] Power [Member]		Dec. 31, 2012 Other [Member] Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2011 Other [Member] Regulatory Liabilities Or Assets [Member] Fuel Oils [Member]		Dec. 31, 2012 Other [Member] Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2011 Other [Member] Regulatory Liabilities Or Assets [Member] Natural Gas [Member]		Dec. 31, 2012 Other [Member] Regulatory Liabilities Or Assets [Member] Uranium [Member]		Dec. 31, 2011 Other [Member] Regulatory Liabilities Or Assets [Member] Uranium [Member]		Apr. 30, 2008 Interest Rate Swap [Member]	Jun. 30, 2002 Interest Rate Swap [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)										$ 47	[1]	$ 19	[1]	$ (7)	[2],[3]	$ (8)	[2],[3]	$ (99)	[4]	$ 81	[4]	$ 4	[5]	$ 19	[5]	$ (107)	[6]	$ (191)	[6]	$ (2)	[7]	$ (1)	[7]								[1]		[1]		[2],[3]		[2],[3]	$ 12	[4]	$ 21	[4]	$ 4	[5]	$ 19	[5]	$ (14)	[6]	$ (24)	[6]	$ (2)	[7]	$ (1)	[7]						[1]		[1]		[2],[3]		[2],[3]	$ (111)	[4]	$ (140)	[4]		[5]		[5]	$ (93)	[6]	$ (167)	[6]		[7]		[7]	$ 47	[1],[8]	$ 19	[1],[8]	$ (7)	[2],[3],[8]	$ (8)	[2],[3],[8]		[4],[8]	$ 200	[4],[8]		[5],[8]		[5],[8]		[6],[8]		[6],[8]		[7],[8]		[7],[8]
Gain (loss) to be reclassified within twelve months			32.0					(1.4)
Derivative, term of contract																																																																																																																					10 years
Debt Instrument, term																																																																																																																					30 years
Gain (loss) in other comprehensive income (loss), amortization period																																																																																																																				10 years	10 years
Carrying value of net losses associated with interest rate swaps				1				8	9
Current gains deferred as regulatory liabilities	100	133	14		4	1																												18	57	14	4																											82	76		1
Current losses deferred as regulatory assets	$ 247	$ 215	$ 24		$ 1	$ 64	$ 1																											$ 163	$ 109	$ 3	$ 1	$ 8	$ 1																									$ 84	$ 306	$ 21	$ 56

[1] Represents net gains associated with power derivative contracts at Ameren. These contracts are a partial hedge of electricity price exposure through December 2016 as of December 31, 2012. In light of market prices at December 31, 2012, net pretax unrealized gains of $32 million are expected to be reclassified into earnings during the next 12 months as the hedged transaction occur. However, the actual amount reclassified from accumulated OCI could vary due to future changes in market prices.

[2] Includes net gains associated with interest rate swaps at Genco that were a partial hedge of the interest rate on debt issued in June 2002. The swaps covered the first 10 years of debt that has a 30-year maturity, and the gain in OCI was amortized over a 10-year period that began in June 2002. The balance of the gain was fully amortized as of June 30, 2012. The carrying value at December 31, 2011, was less than $1 million.

[3] Includes net losses associated with interest rate swaps at Genco. The swaps were executed during the fourth quarter of 2007 as a partial hedge of interest rate risks associated with Genco's April 2008 debt issuance. The loss on the interest rate swaps is being amortized over a 10-year period that began in April 2008. The carrying value at December 31, 2012, and December 31, 2011, was a loss of $8 million and $9 million, respectively. Over the next twelve months ending December 31, 2013, $1.4 million of the loss will be amortized.

[4] Represents net losses associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2015 at Ameren Missouri, in each case as of December 31, 2012. Current gains deferred as regulatory liabilities include $14 million and $14 million at Ameren and Ameren Missouri, respectively, as of December 31, 2012. Current losses deferred as regulatory assets include $24 million, $3 million, and $21 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2012.

[5] Represents net gains on fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through October 2015 as of December 31, 2012. Current gains deferred as regulatory liabilities include $4 million and $4 million at Ameren and Ameren Missouri as of December 31, 2012, respectively. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri as of December 31, 2012, respectively.

[6] Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through March 2017 at Ameren and Ameren Missouri and through October 2016 at Ameren Illinois, in each case as of December 31, 2012. Current gains deferred as regulatory liabilities include $1 million and $1 million at Ameren and Ameren Illinois, respectively, as of December 31, 2012. Current losses deferred as regulatory assets include $64 million, $8 million, and $56 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2012.

[7] Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri's uranium requirements through September 2014 as of December 31, 2012. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri as of December 31, 2012, respectively.

[8] Includes amounts for Marketing Company, Genco, and intercompany eliminations