Schedule II - Valuation And Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 17		$ 20		$ 23
Charged to Costs and Expenses	35		30		41
Charged to Other Accounts	4	[1]	2	[1]		[1]
Deductions	38	[2]	35	[2]	44	[2]
Balance at End of Period	18		17		20
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	2		1		1
Charged to Costs and Expenses	5		1
Charged to Other Accounts
Deductions
Balance at End of Period	7		2		1
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	5		7		8
Charged to Costs and Expenses	16		11		17
Charged to Other Accounts		[1]		[1]		[1]
Deductions	16	[2]	13	[2]	18	[2]
Balance at End of Period	5		5		7
Union Electric Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	1		1		1
Charged to Costs and Expenses
Charged to Other Accounts
Deductions
Balance at End of Period	1		1		1
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	12		13		13
Charged to Costs and Expenses	19		19		24
Charged to Other Accounts	4	[1]	2	[1]		[1]
Deductions	22	[2]	22	[2]	24	[2]
Balance at End of Period	13		12		13
Ameren Illinois Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	1
Charged to Costs and Expenses			1
Charged to Other Accounts
Deductions
Balance at End of Period	$ 1		$ 1

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

[2] Uncollectible accounts charged off, less recoveries.