Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Line Items]
Revenues	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 1,399	$ 1,723	$ 1,537	$ 1,515	$ 6,291	$ 6,174	$ 6,076
Deferred Federal Income Tax Expense (Benefit)									220	511	299
Operating Income	166	533	385	273	212	569	387	242	1,357	1,410	1,322
Net income	69	359	240	153	(59)	290	194	104	821	529	659
Net Income Available to Common Shareholder	$ 68	$ 357	$ 239	$ 151	$ (60)	$ 288	$ 193	$ 102	$ 815	$ 523	$ 653
Earnings per Common Share – Basic (in dollars per share)	$ 0.28	$ 1.46	$ 0.98	$ 0.62	$ (0.24)	$ 1.19	$ 0.79	$ 0.42	$ 3.34	$ 2.16	$ 2.69
Earnings per Common Share – Diluted (in dollars per share)	$ 0.28	$ 1.45	$ 0.97	$ 0.62	$ (0.24)	$ 1.18	$ 0.79	$ 0.42	$ 3.32	$ 2.14	$ 2.68
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Revenues	$ 713	$ 1,129	$ 955	$ 792	$ 696	$ 1,116	$ 934	$ 791
Deferred Federal Income Tax Expense (Benefit)									$ 22	$ 76	$ 161
Operating Income	7	394	258	90	33	412	230	47	749	722	725
Net income	(22)	295	169	39	(36)	235	121	6	481	326	360
Net Income Available to Common Shareholder	(22)	294	168	38	(36)	234	120	5
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Revenues	674	564	578	760	674	574	576	703	2,576	2,527	2,489
Deferred Federal Income Tax Expense (Benefit)									75	185	117
Operating Income	135	113	105	159	148	124	128	169	512	569	519
Net income	85	63	63	96	78	55	58	80	$ 307	271	$ 255
Net Income Available to Common Shareholder	$ 84	$ 63	$ 62	$ 95	$ 77	$ 55	$ 57	$ 79
Tax Cuts and Jobs Act [Member]
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)										154
Tax Cuts and Jobs Act [Member] | Union Electric Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)										32
Tax Cuts and Jobs Act [Member] | Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)										$ (5)