Other Income, Net (Other Income And Expenses) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		$ 36	$ 24	$ 27
Interest income on industrial development revenue bonds		26	26	27
Interest and dividend income		7	8	13
Defined Benefit Plan, Non-service Cost or Income Components		70	44	56
Other		8	5	10
Donations		(33)	(8)	(16)
Other		(12)	(13)	(16)
Other Nonoperating Income (Expense)		102	86	101
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	$ 17
Allowance for equity funds used during construction		27	21	23
Interest income on industrial development revenue bonds		26	26	27
Interest and dividend income		2	1	1
Defined Benefit Plan, Non-service Cost or Income Components		17	22	18
Other		4	3	3
Donations		(14)	(2)	(4)
Other		(6)	(6)	(6)
Other Nonoperating Income (Expense)		56	65	62
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		9	3	4
Interest and dividend income		6	7	12
Defined Benefit Plan, Non-service Cost or Income Components		34	10	24
Other		3	2	6
Donations		(6)	(5)	(6)
Other		(4)	(5)	(6)
Other Nonoperating Income (Expense)		$ 42	$ 12	$ 34