Rate And Regulatory Matters (Narrative) (Details) (USD $)	1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended					1 Months Ended	12 Months Ended	60 Months Ended	12 Months Ended	1 Months Ended		12 Months Ended					12 Months Ended											12 Months Ended		12 Months Ended															1 Months Ended
	Nov. 30, 2012 design	Dec. 31, 2013 design		Dec. 31, 2012		Dec. 31, 2011		Jun. 30, 2008 FERC Relicensing Taum Sauk Energy Center	Dec. 31, 2013 Union Electric Company	Dec. 31, 2012 Union Electric Company	Dec. 31, 2011 Union Electric Company	Dec. 31, 2013 Union Electric Company New Nuclear Energy Center COL	Feb. 13, 2014 Union Electric Company Electric Distribution customer	Apr. 30, 2011 Union Electric Company Fac Prudence Review	Dec. 31, 2012 Union Electric Company Entergy Refund	Dec. 31, 2005 Union Electric Company Pending Ferc Case [Member] Power Purchase Agreement With Entergy Arkansas	Dec. 31, 2012 Union Electric Company Final Rate Order Electric Distribution	Dec. 31, 2012 Union Electric Company Final Rate Order MEEIA Electric Distribution	Dec. 31, 2013 Union Electric Company Accounting Authority Order Request Electric Distribution	Dec. 31, 2013 Ameren Illinois Company customer	Dec. 31, 2012 Ameren Illinois Company	Dec. 31, 2011 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company Wholesale Distribution Rate Case	Dec. 31, 2012 Ameren Illinois Company Wholesale Distribution Rate Case	Dec. 31, 2013 Ameren Illinois Company Final Rate Order Gas Distribution	Dec. 18, 2013 Ameren Illinois Company Final Rate Order Gas Distribution	Dec. 09, 2013 Ameren Illinois Company Final Rate Order Electric Distribution	Dec. 09, 2013 Ameren Illinois Company Final Rate Order IEIMA Electric Distribution	Dec. 31, 2012 Ameren Illinois Company Final Rate Order IEIMA Electric Distribution	Dec. 09, 2013 Ameren Illinois Company IEMA Revenue Requirement Reconciliation Electric Distribution	Dec. 31, 2013 Ameren Illinois Company IEMA Revenue Requirement Reconciliation IEIMA Electric Distribution	Dec. 31, 2012 Ameren Illinois Company IEMA Revenue Requirement Reconciliation IEIMA Electric Distribution	Dec. 31, 2013 ATXI Potential Transmission Project Investments Through 2019	Dec. 31, 2013 Maximum Union Electric Company New Nuclear Energy Center COL	Dec. 31, 2013 Maximum Ameren Illinois Company Pending Ferc Case [Member]	Dec. 31, 2013 Minimum Union Electric Company New Nuclear Energy Center COL	Dec. 31, 2013 Pending Ferc Case [Member] Ameren Illinois Company customer	Dec. 31, 2013 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member]	Dec. 31, 2013 Fac Prudence Review Union Electric Company Subsequent Periods After September 30, 2009	Dec. 31, 2013 FERC Revenue Requirement Reconciliation		Dec. 31, 2012 FERC Revenue Requirement Reconciliation		Dec. 31, 2013 FERC Revenue Requirement Reconciliation Union Electric Company		Dec. 31, 2012 FERC Revenue Requirement Reconciliation Union Electric Company		Dec. 31, 2013 FERC Revenue Requirement Reconciliation Ameren Illinois Company		Dec. 31, 2012 FERC Revenue Requirement Reconciliation Ameren Illinois Company		Dec. 31, 2013 FERC Revenue Requirement Reconciliation ATXI	Feb. 28, 2014 Customer [Domain] Union Electric Company Electric Distribution
Rate And Regulatory Matters [Line Items]
Refund to Customers Under FAC														$ 18,000,000
Authorized increase in revenue from utility service																	260,000,000									32,000,000
Loss Contingency, Settlement Agreement, Number of Wholesale Customers																																					4
Number of Wholesale Customers																				9																	5
Rate of return on common equity																	9.80%								9.10%													12.38%
Percent of capital structure composed of equity																	52.30%								51.70%
Rate base																	6,800,000,000								1,100,000,000
Incentive Award if Energy Efficiency Goals Are Achieved																		19,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award																		100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period																		3 years
Minimum Percentage of Energy Efficiency Goal Achievement For Company To Be Eligible For Incentive Award																		70.00%
Number of Customers Filed Complaint Case													37
Contested return on equity																																																					9.80%
Customer Requested Rate on Equity																																																					9.40%
Revenue Requirement																												788,000,000	765,000,000
Authorized Decrease In Revenue From Utility Service																											(45,000,000)	23,000,000		(68,000,000)
Disallowed costs associated with debt redemption																										15,000,000	15,000,000
Current regulatory liabilities		216,000,000		100,000,000					57,000,000	18,000,000										159,000,000	82,000,000		13,000,000	8,000,000							65,000,000
Noncurrent regulatory liabilities		1,705,000,000		1,589,000,000					1,041,000,000	917,000,000										664,000,000	672,000,000											55,000,000								10,000,000	[1]	0	[1]	0	[1]	0	[1]	10,000,000	[1]	0	[1]
Loss Contingency, Estimate of Possible Loss																																			15,000,000
Proceeds from Legal Settlements															31,000,000
Purchased power		502,000,000		780,000,000		952,000,000			127,000,000	78,000,000	104,000,000				24,000,000	25,000,000				380,000,000	705,000,000	853,000,000
Department of Energy, Investing Funding Support, Number of Small Modular Reactor Designs		2
Department of Energy, Investing Funding Support, Period		5 years
Number Of Years COL is Valid For																																				40 years
Department of Energy, Investing Funding Support, Number of Small Modular Reactor Designs Awarded	1
Other Nonoperating Income		69,000,000	[2]	70,000,000	[2]	68,000,000	[2]		58,000,000	63,000,000	61,000,000				5,000,000					10,000,000	7,000,000	7,000,000
Reduction To Under-recovered Asset															2,000,000
Customer Refund Liability, Current																																							26,000,000
Interest Expense		398,000,000		392,000,000		387,000,000			210,000,000	223,000,000	209,000,000			1,000,000						143,000,000	129,000,000	136,000,000																	1,000,000
Noncurrent regulatory assets		1,240,000,000		1,786,000,000					534,000,000	852,000,000									36,000,000	701,000,000	934,000,000										65,000,000																					5,000,000
Capital investments												$ 69,000,000																					$ 1,100,000,000	$ 100,000,000		$ 80,000,000
Number of years for proposed relicensing application filed with FERC								40 years

[1] Ameren Illinois' 2013 revenue requirement reconciliation adjustment calculated pursuant to the FERC's electric transmission formula ratemaking framework. This liability will be refunded to customers in 2015.

[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.