Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	$ 650,000,000	$ 623,000,000
Liabilities incurred	0	3,000,000
Liabilities settled	(13,000,000)	(2,000,000)
Accretion in period	26,000,000	25,000,000
Change in estimates	(19,000,000)	1,000,000
Balance	644,000,000	650,000,000	$ 623,000,000
Other current liabilities	326,000,000	274,000,000
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	6,000,000	15,000,000
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	644,000,000	617,000,000
Liabilities incurred		3,000,000
Liabilities settled	(12,000,000)	(2,000,000)
Accretion in period	26,000,000	25,000,000
Change in estimates	(18,000,000)	1,000,000
Balance	640,000,000	644,000,000	617,000,000
Other current liabilities	103,000,000	123,000,000
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	6,000,000	6,000,000
Liabilities incurred	0	0
Liabilities settled	(1,000,000)	(1,000,000)
Accretion in period	1,000,000	1,000,000
Change in estimates	(1,000,000)
Balance	4,000,000	6,000,000	6,000,000
Other current liabilities	177,000,000	109,000,000
Asset Retirement Obligation | Union Electric Company
Asset Retirement Obligation [Line Items]
Noncash Depreciation related to ARO	$ 26,000,000	$ 31,000,000	$ 13,000,000