Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Mar. 31, 2014	Dec. 31, 2010
Accounting Policies [Line Items]
Unrecognized Tax Benefits	$ 90	[1]	$ 156	[1]	$ 148	[1]	$ 48	$ 246	[1]
Goodwill	411		411
Public Utilities, Property, Plant and Equipment, Amount of Indirect Disallowance of Costs of Recently Completed Plants					89
Tax grants received related to renewable energy properties			18
Book value	22		14
Ameren Illinois Company
Accounting Policies [Line Items]
Unrecognized Tax Benefits	(1)		13		11			56
Goodwill	411		411
Public Utilities, Area Serviced	40,000
Public Utilities, Estimated Population of Service Territory	3,100,000
Union Electric Company
Accounting Policies [Line Items]
Unrecognized Tax Benefits	31		136		124		15	164
Public Utilities, Property, Plant and Equipment, Amount of Indirect Disallowance of Costs of Recently Completed Plants					89
Tax grants received related to renewable energy properties			18
Public Utilities, Area Serviced	24,000
Public Utilities, Estimated Population of Service Territory	2,800,000
Book value	22		14
Electric Energy Inc
Accounting Policies [Line Items]
Percentage of EEI not owned by Ameren			20.00%
Voluntary Separation Offer
Accounting Policies [Line Items]
Number Of Employee Positions Eliminated					340
Severance Costs					28
Voluntary Separation Offer | Union Electric Company
Accounting Policies [Line Items]
Severance Costs					27
Minimum
Accounting Policies [Line Items]
Percent of average depreciable cost	3.00%		3.00%		3.00%
Maximum
Accounting Policies [Line Items]
Percent of average depreciable cost	4.00%		4.00%		4.00%
Power | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	1,200,000
Power | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	1,200,000
Natural Gas | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	767,000
Natural Gas | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	127,000
FAC | Union Electric Company
Accounting Policies [Line Items]
Sharing Level For Fac	95.00%
Taum Sauk Energy Center | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Property, Plant and Equipment, Amount of Indirect Disallowance of Costs of Recently Completed Plants					$ 89

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.