Long-Term Debt And Equity Financings (Narrative) (Detail) (USD $)	9 Months Ended									1 Months Ended		1 Months Ended				1 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Union Electric Company	Sep. 30, 2013 Union Electric Company	Sep. 30, 2014 Ameren Missouri And Ameren Illinois [Member]	Sep. 30, 2014 Ameren Illinois Company	Sep. 30, 2013 Ameren Illinois Company	Sep. 30, 2014 Ameren Illinois Company Federal Energy Regulatory Commission Restriction [Member]	Sep. 30, 2014 Ameren Illinois Company Minimum Federal Energy Regulatory Commission Restriction [Member]	May 31, 2014 Senior Unsecured Notes8875 Due2014 [Member] Parent Company [Member]	May 15, 2014 Senior Unsecured Notes8875 Due2014 [Member] Parent Company [Member]	Jan. 31, 2014 Environmental Improvement And Pollution Control Revenue Bonds [Member] Ameren Illinois Company	Apr. 30, 2014 Secured Debt [Member] Senior Secured Notes, 3.50%, Due 2024 [Member] Union Electric Company	May 31, 2014 Secured Debt [Member] Senior Secured Notes550 Due2014 [Member] Union Electric Company	May 15, 2014 Secured Debt [Member] Senior Secured Notes550 Due2014 [Member] Union Electric Company	Jun. 30, 2014 Secured Debt [Member] Senior Secured Notes, 4.30%, Due 2044 [Member] [Member] Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Repayments of Other Long-term Debt	$ 692,000,000									$ 425,000,000
Debt Instrument, Face Amount						1,000,000							350,000,000			250,000,000
Redemptions of Long-term Debt			104,000,000			163,000,000						163,000,000		104,000,000
Debt Instrument, Interest Rate, Stated Percentage											8.875%		3.50%		5.50%	4.30%
Assumed interest rate					5.00%
Dividend rate					6.00%
Common stock equity to total capitalization								54.00%	30.00%
Proceeds from Issuance of Secured Debt													$ 348,000,000			$ 246,000,000