Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 1,316	$ 1,659	$ 1,379	$ 1,556	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 5,910	$ 6,291	$ 6,174
Operating Income	179	520	280	288	166	533	385	273	1,267	1,357	1,410
Net Income (Loss)	95	366	180	193	69	359	240	153	834	821	529
Net Income (Loss) Available to Common Shareholder	$ 94	$ 364	$ 179	$ 191	$ 68	$ 357	$ 239	$ 151	$ 828	$ 815	$ 523
Earnings per common share – basic	$ 0.38	$ 1.48	$ 0.73	$ 0.78	$ 0.28	$ 1.46	$ 0.98	$ 0.62	$ 3.37	$ 3.34	$ 2.16
Earnings per common share – diluted	$ 0.38	$ 1.47	$ 0.72	$ 0.78	$ 0.28	$ 1.45	$ 0.97	$ 0.62	$ 3.35	$ 3.32	$ 2.14
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 628	$ 1,059	$ 798	$ 758	$ 713	$ 1,129	$ 955	$ 792	$ 3,243	$ 3,589	$ 3,537
Operating Income	5	381	152	79	7	394	258	90	617	749	722
Net Income (Loss)	(20)	301	108	40	(22)	295	169	39	429	481	326
Net Income (Loss) Available to Common Shareholder	(20)	300	107	39	(22)	294	168	38
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	654	564	547	762	674	564	578	760	2,527	2,576	2,527
Operating Income	150	110	104	186	135	113	105	159	550	512	569
Net Income (Loss)	97	65	63	121	85	63	63	96	$ 346	$ 307	$ 271
Net Income (Loss) Available to Common Shareholder	$ 96	$ 65	$ 62	$ 120	$ 84	$ 63	$ 62	$ 95