Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 21	$ 18	$ 17
Charged to Costs and Expenses		33	36	35
Charged to Other Accounts	[1]	5	4	4
Deductions	[2]	40	37	38
Balance at End of Period		19	21	18
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		10	7	2
Charged to Costs and Expenses		4	3	5
Charged to Other Accounts	[1]	(8)	0	0
Deductions		0	0	0
Balance at End of Period		6	10	7
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		8	5	5
Charged to Costs and Expenses		13	16	16
Charged to Other Accounts	[1]	0	0	0
Deductions	[2]	14	13	16
Balance at End of Period		7	8	5
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		0	0	0
Charged to Other Accounts	[1]	(1)	0	0
Deductions		0	0	0
Balance at End of Period		0	1	1
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		13	13	12
Charged to Costs and Expenses		20	20	19
Charged to Other Accounts	[1]	5	4	4
Deductions	[2]	26	24	22
Balance at End of Period		12	13	13
Ameren Illinois 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		0	0	0
Charged to Other Accounts	[1]	(1)	0	0
Deductions		0	0	0
Balance at End of Period		$ 0	$ 1	$ 1

[1] Amounts associated with the allowance for doubtful accounts relate to the uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers, as required by the Illinois Public Utilities Act. The amounts relating to the deferred tax valuation allowance are for items that have expired and were removed from both the underlying accumulated deferred income tax account as well as the offsetting valuation account.

[2] Uncollectible accounts charged off, less recoveries.