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Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accounting Policies [Line Items]
Nuclear decommissioning trust fund	$ 549us-gaap_DecommissioningFundInvestments	$ 494us-gaap_DecommissioningFundInvestments
Average performance share units excluded from calculation	0	0	0
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	52	84
Unrecognized Tax Benefits	54us-gaap_UnrecognizedTaxBenefits	90us-gaap_UnrecognizedTaxBenefits	156us-gaap_UnrecognizedTaxBenefits	148us-gaap_UnrecognizedTaxBenefits
Goodwill	411us-gaap_Goodwill	411us-gaap_Goodwill
Ameren Illinois Company
Accounting Policies [Line Items]
Capital contribution from parent	15
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	0	0
Unrecognized Tax Benefits	(1)	(1)	13	11
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]
Capital contribution from parent	215	4	1
Noncash Or Part Noncash Capital Contribution From Parent	9
Nuclear decommissioning trust fund	549	494
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	0	15
Unrecognized Tax Benefits	0	31	136	124
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
Return of capital to parent	$ (215)
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	813,000
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%