Short-Term Debt And Liquidity (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) lender	Dec. 31, 2016
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,600,000,000
Covenant terms, default provisions, maximum indebtedness	100,000,000
Multiyear Credit Facility
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,100,000,000
Number of lenders | lender	22
Line of credit facility, maximum borrowing capacity, per lender	$ 118,000,000
Actual debt-to-capital ratio	0.53
Line of Credit Facility, Commitment Fee Amount	$ 100,000,000
Multiyear Credit Facility | Maximum
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.65
Illinois Credit Agreement 2012 | Maximum
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000
Missouri Credit Agreement 2012 | Maximum
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	1,200,000,000
Parent Company | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	500,000,000
Parent Company | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	700,000,000
Union Electric Company | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.48
Ameren Illinois Company | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.47
Utilities [Member]
Short-term Debt [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.19%	0.52%