Schedule II - Valuation And Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Allowance For Doubtful Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 20		$ 23		$ 24
Charged to Costs and Expenses	30		41		33
Charged to Other Accounts	2	[1]		[1]		[1]
Deductions	35	[2]	44	[2]	34	[2]
Balance at End of Period	17		20		23
Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	2		2
Charged to Costs and Expenses	2				2
Charged to Other Accounts
Deductions
Balance at End of Period	4		2		2
Union Electric Company [Member] | Allowance For Doubtful Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	7		8		6
Charged to Costs and Expenses	11		17		14
Charged to Other Accounts		[1]		[1]		[1]
Deductions	13	[2]	18	[2]	12	[2]
Balance at End of Period	5		7		8
Union Electric Company [Member] | Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	1		1
Charged to Costs and Expenses					1
Charged to Other Accounts
Deductions
Balance at End of Period	1		1		1
Ameren Illinois Company [Member] | Allowance For Doubtful Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	13		13		17
Charged to Costs and Expenses	19		24		18
Charged to Other Accounts	2	[1]		[1]		[1]
Deductions	22	[2]	24	[2]	22	[2]
Balance at End of Period	12		13		13
Ameren Illinois Company [Member] | Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period
Charged to Costs and Expenses	1
Charged to Other Accounts
Deductions
Balance at End of Period	$ 1

[1] Uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers as required by the Illinois Public Utility Act.

[2] Uncollectible accounts charged off, less recoveries.