Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)							3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Ameren (parent) [Domain]	Dec. 31, 2013 Ameren (parent) [Domain]	Mar. 31, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Mar. 31, 2014 Utilities [Member]	Mar. 31, 2013 Utilities [Member]	Mar. 31, 2014 Credit Agreements 2012 [Member]	Mar. 31, 2014 Credit Agreements 2012 [Member] Maximum	Mar. 31, 2014 Missouri Credit Agreement 2012 [Member] Union Electric Company	Mar. 31, 2014 Illinois Credit Agreement 2012 [Member] Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity									$ 1,400,000,000
Commercial paper outstanding	$ 700,000,000	$ 368,000,000	$ 410,000,000	$ 368,000,000	$ 290,000,000
Actual debt-to-capital ratio									0.50	0.65	0.49	0.44
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date									2.0 to 1.0
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date									5.6 to 1.0
Short Term Debt, Weighted Average Interest Rate During Period							0.39%	0.11%