Rate And Regulatory Matters (Narrative-Illinois) (Detail) (USD $)					12 Months Ended		3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company Gas Distribution [Member] Final Rate Order [Member]	Dec. 31, 2013 Ameren Illinois Company Gas Distribution [Member] Rate order appeal [Member]	Mar. 31, 2014 Ieima [Member] Ameren Illinois Company Electric Distribution [Member] IEMA Revenue Requirement Reconciliation [Member]	Mar. 31, 2014 Pending Ferc Case [Member] Midwest Independent Transmission System Operator, Inc [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,251,000,000	$ 1,240,000,000	$ 706,000,000	$ 701,000,000			$ 24,000,000
Requested rate decrease, amount							206,000,000
Authorized Increase in Revenue from Utility Service					32,000,000
Rate Of Return On Common Equity					9.10%	10.40%		12.38%
Percent Of Capital Structure Composed Of Equity					51.70%
Rate Base					$ 1,100,000,000