Summary Of Significant Accounting Policies (Basic and Diluted Earnings Per Share Calculations) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Accounting Policies [Abstract]
Continuing Operations	$ 48	$ 305	$ 105	$ 54	$ 12	$ 302	$ 164	$ 38	$ 512		$ 516		$ 431
Discontinued Operations	11	3	10	199	1,168	(72)	(47)	441	(223)		(1,490)		88
Net income (loss) attributable to Ameren Corporation	$ 37	$ 302	$ 95	$ (145)	$ (1,156)	$ 374	$ 211	$ (403)	$ 289		$ (974)		$ 519
Average Common Shares Outstanding - Basic									242.6		242.6		241.5
Assumed Settlement of Performance Share Units									1.9		0.4		0.6
Average Common Shares Outstanding - Diluted									244.5		243.0		242.1
Continuing Operations - Basic	$ 0.19	$ 1.26	$ 0.44	$ 0.22	$ 0.05	$ 1.25	$ 0.67	$ 0.16	$ 2.11		$ 2.13		$ 1.79
Discontinued Operations - Basic	$ (0.04)	$ (0.01)	$ (0.05)	$ (0.82)	$ (4.81)	$ 0.29	$ 0.20	$ (1.82)	$ (0.92)		$ (6.14)		$ 0.36
Earnings (Loss) per Common Share – Basic	$ 0.15	$ 1.25	$ 0.39	$ (0.60)	$ (4.76)	$ 1.54	$ 0.87	$ (1.66)	$ 1.19		$ (4.01)		$ 2.15
Continuing Operations - Diluted	$ 0.19	$ 1.25	$ 0.44	$ 0.22	$ 0.05	$ 1.25	$ 0.67	$ 0.16	$ 2.10		$ 2.13		$ 1.79
Discontinued Operations - Diluted	$ (0.04)	$ (0.01)	$ (0.05)	$ (0.82)	$ (4.81)	$ 0.29	$ 0.20	$ (1.82)	$ (0.92)		$ (6.14)		$ 0.36
Earnings (Loss) per Common Share – Diluted	$ 0.15	$ 1.24	$ 0.39	$ (0.60)	$ (4.76)	$ 1.54	$ 0.87	$ (1.66)	$ 1.18		$ (4.01)		$ 2.15
Average performance share units excluded from calculation									0.1	[1]	0.7	[1]	0	[1]

[1] Weighted-average number of performance share units that were excluded from the “Assumed settlement of performance share units” provided above because the performance or market conditions related to the awards had not yet been met.