Rate And Regulatory Matters (Narrative-Missouri) (Detail) - Electric Distribution - Union Electric Company $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Final Rate Order
Rate And Regulatory Matters [Line Items]
Revenue Requirement	$ 3,400
Public Utilities, Requested Rate Increase (Decrease), Amount	92
Components of Rate Increase - Net Energy Costs
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	54
Components of Rate Increase - Regulatory Amortizations
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 26