Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Missouri) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		$ 85
Ameren Transmission Company of Illinois | Mark Twain Project
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		250
Final Rate Order | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Revenue Requirement		3,400
Public Utilities, Requested Rate Increase (Decrease), Amount		92
Final Rate Order | Union Electric Company | MEEIA 2013 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		28
Final Rate Order | Union Electric Company | MEEIA 2016 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		$ 27
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%
Pending Rate Case | Union Electric Company | MEEIA 2013 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		$ 9
Components of Rate Increase - Net Energy Costs [Member] | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		54
Components of Rate Increase - Regulatory Amortizations [Member] | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 26
Subsequent Event [Member] | Final Rate Order | Union Electric Company | MEEIA 2016 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved	$ 5