Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $) In Millions, unless otherwise specified						1 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Mar. 31, 2014 Electric Distribution [Member] Union Electric Company Accounting Authority Order Request [Member]	Feb. 28, 2014 Customer [Domain] Electric Distribution [Member] Union Electric Company	Feb. 13, 2014 Customer [Domain] Electric Distribution [Member] Union Electric Company customer
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,251	$ 1,240	$ 539	$ 534	$ 36
Number of Customers Filed Complaint Case							37
Contested return on equity						9.80%
Customer Requested Rate Decrease						$ 67
Customer Requested Rate on Equity						9.40%