Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Weighted Average Number of Common Shares Outstanding) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Continuing Operations	$ 305		$ 309		$ 464		$ 507
Discontinued Operations	(3)		65		(212)		(325)
Net Income (Loss) Attributable to Ameren Corporation	$ 302		$ 374		$ 252		$ 182
Average Common Shares Outstanding - Basic	242.6		242.6		242.6		242.6
Assumed Settlement of Performance Share Units	2.5		0.3		1.8		0.3
Average Common Shares Outstanding - Diluted	245.1		242.9		244.4		242.9
Continuing Operations - Basic	$ 1.26		$ 1.28		$ 1.92		$ 2.09
Discontinued Operations - Basic	$ (0.01)		$ 0.26		$ (0.88)		$ (1.34)
Earnings per Common Share - Basic	$ 1.25		$ 1.54		$ 1.04		$ 0.75
Continuing Operations - Diluted	$ 1.25		$ 1.28		$ 1.91		$ 2.09
Discontinued Operations - Diluted	$ (0.01)		$ 0.26		$ (0.88)		$ (1.34)
Earnings per Common Share - Diluted	$ 1.24		$ 1.54		$ 1.03		$ 0.75
Average Performance Share Units Excluded From Calculation	0	[1]	1.0	[1]	0.1	[1]	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.