Schedule II - Valuation And Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		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	$ 18		$ 17		$ 20
Charged to Costs and Expenses	36		35		30
Charged to Other Accounts	4	[1]	4	[1]	2	[1]
Deductions	37	[2]	38	[2]	35	[2]
Balance at End of Period	21		18		17
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	7		2		1
Charged to Costs and Expenses	3		5		1
Balance at End of Period	10		7		2
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	5		5		7
Charged to Costs and Expenses	16		16		11
Deductions	13	[2]	16	[2]	13	[2]
Balance at End of Period	8		5		5
Union Electric Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period							1
Balance at End of Period	1		1		1		1
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	13		12		13
Charged to Costs and Expenses	20		19		19
Charged to Other Accounts	4	[1]	4	[1]	2	[1]
Deductions	24	[2]	22	[2]	22	[2]
Balance at End of Period	13		13		12
Ameren Illinois Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Costs and Expenses					1
Balance at End of Period	$ 1		$ 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.