Rate And Regulatory Matters (Narrative-Missouri) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Rate And Regulatory Matters [Line Items]
Electric	$ 1,223	$ 1,207
Union Electric Company
Rate And Regulatory Matters [Line Items]
Electric	741	$ 747
MEEIA 2016 [Member] | Final Rate Order | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved	$ 27
Achieved Percentage of Energy Efficiency Earnings For Incentive Award	100.00%
Electric	$ 5