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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Continuing Operations	$ 46	$ 294	$ 150	$ 97	$ 48	$ 305	$ 105	$ 54	$ 587	$ 512	$ 516
Discontinued Operations	2	(1)	(1)	(1)	(11)	(3)	(10)	(199)	(1)	(223)	(1,490)
Net income (loss) attributable to Ameren Corporation	$ 48us-gaap_NetIncomeLoss	$ 293us-gaap_NetIncomeLoss	$ 149us-gaap_NetIncomeLoss	$ 96us-gaap_NetIncomeLoss	$ 37us-gaap_NetIncomeLoss	$ 302us-gaap_NetIncomeLoss	$ 95us-gaap_NetIncomeLoss	$ (145)us-gaap_NetIncomeLoss	$ 586us-gaap_NetIncomeLoss	$ 289us-gaap_NetIncomeLoss	$ (974)us-gaap_NetIncomeLoss
Average Common Shares Outstanding - Basic									242.6	242.6	242.6
Assumed Settlement of Performance Share Units									1.8	1.9	0.4
Average Common Shares Outstanding - Diluted									244.4	244.5	243.0
Continuing Operations - Basic	$ 0.19	$ 1.21	$ 0.62	$ 0.40	$ 0.19	$ 1.26	$ 0.44	$ 0.22	$ 2.42	$ 2.11	$ 2.13
Discontinued Operations - Basic	$ 0.01		$ (0.01)	$ 0.00	$ (0.04)	$ (0.01)	$ (0.05)	$ (0.82)	$ 0.00	$ (0.92)	$ (6.14)
Earnings (Loss) per Common Share – Basic	$ 0.20us-gaap_EarningsPerShareBasic	$ 1.21us-gaap_EarningsPerShareBasic	$ 0.61us-gaap_EarningsPerShareBasic	$ 0.40us-gaap_EarningsPerShareBasic	$ 0.15us-gaap_EarningsPerShareBasic	$ 1.25us-gaap_EarningsPerShareBasic	$ 0.39us-gaap_EarningsPerShareBasic	$ (0.60)us-gaap_EarningsPerShareBasic	$ 2.42us-gaap_EarningsPerShareBasic	$ 1.19us-gaap_EarningsPerShareBasic	$ (4.01)us-gaap_EarningsPerShareBasic
Continuing Operations - Diluted	$ 0.19	$ 1.20	$ 0.62	$ 0.40	$ 0.19	$ 1.25	$ 0.44	$ 0.22	$ 2.40	$ 2.10	$ 2.13
Discontinued Operations - Diluted	$ 0.01	$ 0.00	$ (0.01)	$ 0.00	$ (0.04)	$ (0.01)	$ (0.05)	$ (0.82)	$ 0.00	$ (0.92)	$ (6.14)
Earnings (Loss) per Common Share – Diluted	$ 0.20us-gaap_EarningsPerShareDiluted	$ 1.20us-gaap_EarningsPerShareDiluted	$ 0.61us-gaap_EarningsPerShareDiluted	$ 0.40us-gaap_EarningsPerShareDiluted	$ 0.15us-gaap_EarningsPerShareDiluted	$ 1.24us-gaap_EarningsPerShareDiluted	$ 0.39us-gaap_EarningsPerShareDiluted	$ (0.60)us-gaap_EarningsPerShareDiluted	$ 2.40us-gaap_EarningsPerShareDiluted	$ 1.18us-gaap_EarningsPerShareDiluted	$ (4.01)us-gaap_EarningsPerShareDiluted
Average performance share units excluded from calculation									0	0	0

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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