Long-Term Debt And Equity Financings (Narrative) (Details) (USD $)	12 Months Ended					1 Months Ended			12 Months Ended						12 Months Ended		1 Months Ended	12 Months Ended				1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Senior Unsecured Notes 8.875% Due 2014 [Member]	Sep. 30, 2010 Series 7.69% Due 2036 [Member]	Nov. 30, 2010 Senior Notes Series D 8.35% Due 2010 [Member]	Jun. 30, 2011 Senior Secured Notes 6.625% Due 2011 [Member]	Aug. 31, 2010 Series Preferred Stock $7.64 [Member]	Dec. 31, 2011 401 (K) [Member]	Dec. 31, 2010 401 (K) [Member]	Dec. 31, 2009 401 (K) [Member]	Dec. 31, 2011 DRPlus [Member]	Dec. 31, 2011 Union Electric Company [Member]	Dec. 31, 2010 Union Electric Company [Member]	Dec. 31, 2011 Union Electric Company [Member] Senior Secured Notes 8.45% Due 2039 [Member]	Aug. 31, 2010 Union Electric Company [Member] Series Preferred Stock $7.64 [Member]	Sep. 30, 2010 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Dec. 31, 2010 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member] Senior Notes Series I 6.30% Due 2020 [Member]	Dec. 31, 2011 Ameren Illinois Company [Member] Senior Secured Notes 6.625% Due 2011 [Member]	Aug. 31, 2010 Ameren Illinois Company [Member] Series Preferred Stock 4.50% [Member]	Aug. 31, 2010 Ameren Illinois Company [Member] Series Preferred Stock 4.64% [Member]	Feb. 28, 2010 CILCORP [Member]	Dec. 31, 2011 Ameren Energy Generating Company [Member]	Dec. 31, 2010 Ameren Energy Generating Company [Member]	Dec. 31, 2011 Ameren Energy Generating Company [Member] Senior Notes Series I 6.30% Due 2020 [Member]	Dec. 31, 2010 Ameren Energy Generating Company [Member] Senior Notes Series I 6.30% Due 2020 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	100,000,000
Preferred stock, par value	$ 0.01															$ 7.64
Preferred stock, shares outstanding	0																					111,264
Common stock, shares issued	2,200,000	3,000,000	25,100,000						2,200,000	3,000,000	3,200,000
Common stock, value of shares issued	$ 65,000,000	$ 80,000,000	$ 634,000,000						$ 65,000,000	$ 80,000,000	$ 82,000,000
Debt instrument face amount					66,000,000			33,000,000									40,000,000										250,000,000	250,000,000
Stated interest rate on debt instrument				8.875%	7.69%	8.35%	6.625%								8.45%					6.30%	6.625%
Debt instrument maturity year				2014											2039					2020	2011
Common stock, shares authorized	400,000,000	400,000,000										6,000,000	150,000,000	150,000,000				45,000,000	45,000,000						10,000	10,000
Contributed capital																	33,000,000
Repayments of senior debt							150,000,000
Amount of senior notes matured and retired						200,000,000
Principle amount outstanding													3,955,000,000	3,960,000,000				1,666,000,000	1,816,000,000						825,000,000	825,000,000
Fair-market value adjustments																		5,000,000	5,000,000
Value of cash and securities deposited for covenant defeasance																								3,000,000
Number of preferred stock shares redeemed																							79,940
Preferred stock, redemption price per share																$ 100.85						$ 110	$ 102
Redemption price debt instrument					102.692%												101.52%
Dividend rate on preferred shares, percentage																						4.50%	4.64%
Excess in indebtedness upon default of maturity				$ 25,000,000