Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 19		$ 19	$ 21
Charged to Costs and Expenses		26		32	33
Charged to Other Accounts	[1]	7		3	5
Deductions	[2]	33		35	40
Balance at End of Period		19		19	19
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		11		6	10
Charged to Costs and Expenses		(6)	[3]	7	4
Charged to Other Accounts	[1]	0		(2)	(8)
Deductions		0		0	0
Balance at End of Period		5		11	6
Parent Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Costs and Expenses		3
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		7		7	8
Charged to Costs and Expenses		9		10	13
Charged to Other Accounts	[1]	0		0	0
Deductions	[2]	9		10	14
Balance at End of Period		7		7	7
Union Electric Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		0		0	1
Charged to Costs and Expenses		0		0	0
Charged to Other Accounts	[1]	0		0	(1)
Deductions		0		0	0
Balance at End of Period		0		0	0
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		12		12	13
Charged to Costs and Expenses		17		22	20
Charged to Other Accounts	[1]	7		3	5
Deductions	[2]	24		25	26
Balance at End of Period		12		12	12
Ameren Illinois Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		0		0	1
Charged to Costs and Expenses		0		0	0
Charged to Other Accounts	[1]	0		0	(1)
Deductions		0		0	0
Balance at End of Period		$ 0		$ 0	$ 0

[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.

[3] Includes an adjustment of $3 million to Ameren (parent)’s valuation allowance for certain deferred tax assets existing at December 31, 2017, for the reduction in the income tax rate.