Related Party Transactions (Narrative) (Details) (USD $) In Millions, unless otherwise specified						12 Months Ended	1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Ameren Energy Resources Generating Company [Member]	Dec. 31, 2012 Ameren Energy Marketing Company [Member]	Dec. 31, 2012 Midwest Independent Transmission System Operator, Inc [Member]	May 28, 2012 Ameren Energy Generating Company [Member]	Dec. 31, 2012 Ameren Energy Generating Company [Member] Ameren Transmission Company of Illinois [Member]	Apr. 30, 2011 Ameren Illinois Company [Member] Ameren Transmission Company of Illinois [Member]	Jan. 31, 2011 Ameren Illinois Company [Member] Ameren Transmission Company of Illinois [Member]	Dec. 31, 2012 Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2009 May 31, 2010 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2009 May 31, 2010 [Member] Union Electric Company [Member]	May 31, 2009 May 31, 2010 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Apr. 30, 2010 May 31, 2011 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2009 May 31, 2011 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2009 May 31, 2011 [Member] Union Electric Company [Member]	May 31, 2010 May 31, 2011 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	May 31, 2009 May 31, 2011 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Apr. 30, 2010 May 31, 2012 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2009 May 31, 2012 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2009 May 31, 2012 [Member] Union Electric Company [Member]	May 31, 2011 May 31, 2012 [Member] Ameren Illinois Company [Member] Union Electric Company [Member] MWh	May 31, 2011 May 31, 2012 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	May 31, 2010 May 31, 2012 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Apr. 30, 2012 May 31, 2013 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2010 May 31, 2013 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2010 May 31, 2013 [Member] Union Electric Company [Member]	May 31, 2011 May 31, 2013 [Member] Ameren Illinois Company [Member] Union Electric Company [Member] MWh	May 31, 2011 May 31, 2013 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Apr. 30, 2012 May 31, 2014 [Member] Ameren Energy Marketing Company [Member]	Apr. 30, 2012 May 31, 2014 [Member] Union Electric Company [Member]	May 31, 2011 May 31, 2014 [Member] Ameren Illinois Company [Member] Union Electric Company [Member] MWh	Feb. 29, 2012 May 31, 2014 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	May 31, 2011 May 31, 2014 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Apr. 30, 2012 May 31, 2015 [Member] Union Electric Company [Member]	Feb. 29, 2012 May 31, 2015 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Feb. 29, 2012 May 31, 2016 [Member] Ameren Illinois Company [Member] Ameren Energy Marketing Company [Member] MWh	Dec. 31, 2012 Guarantee Type, Other [Member]
Related Party Transaction [Line Items]
Business Acquisition Put Option Agreement Amount					$ 100
Put Option Premium					2.5
Energy Supply Agreements Amount										4	2		1	9	2			2	8	1				1	3	1			4	1				3
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power												80,000				924,000	89,000				16,800	1,747,000	296,000				40,800	1,841,000			40,800	3,942,000	650,000		3,504,000	1,318,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate												48				33	48				37	37	40				29	42			28	30	42		32	34
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress, Proceeds from Advances to Related Party								52
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress, Proceeds from Advances to Related Party, Accrued Interest								3
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress, Transfers from Related Party							20
Intercompany Payables									5
Financing Receivable, Significant Purchases									35
Related Party Transaction, Purchases of Financing Receivable, Discount (less than $1 million)									1
Related Party Transaction, Proceeds from Sale of Assets						2
Related Party Transaction, Gain (Loss) on Sales of Assets						2
Related Party Transaction, Guarantees Outstanding	354	100	189	50																																	15
Related Party Transaction, Guarantees, Next Twelve Months			161
Related Party Transaction, Guarantees, Year Two			12
Related Party Transaction, Guarantees, Thereafter			16
Related Party Transaction, Guarantees, Maximum Exposure			25	32
Letters of Credit Outstanding, Amount	$ 9