Other Income And Expenses (Other Income And Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 24	$ 27	$ 30
Interest income on industrial development revenue bonds	26	27	27
Interest and dividend income	8	13	14
Other	1	7	3
Total miscellaneous income	59	74	74
Donations	8	16	15
Other	13	16	15
Total miscellaneous expense	21	32	30
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	21	23	22
Interest income on industrial development revenue bonds	26	27	27
Interest and dividend income	1	1	1
Other		1	2
Total miscellaneous income	48	52	52
Donations	2	4	5
Other	6	6	6
Total miscellaneous expense	8	10	11
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	4	8
Interest and dividend income	7	12	12
Other	1	5	1
Total miscellaneous income	11	21	21
Donations	5	6	5
Other	5	6	7
Total miscellaneous expense	$ 10	$ 12	$ 12