Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 30	$ 10	$ 8	$ 9
Restricted Cash and Cash Equivalents	12	21	19	20
Restricted Cash and Cash Equivalents, Noncurrent	44	35	23	22
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	1	1
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	89	68	51	52
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	3	0	0	0
Restricted Cash and Cash Equivalents	4	5	4	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	1	1
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	10	7	5	5
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	3	0	0	0
Restricted Cash and Cash Equivalents	5	6	5	6
Restricted Cash and Cash Equivalents, Noncurrent	44	35	23	22
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	$ 52	$ 41	$ 28	$ 28