Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 6,076	$ 6,098	$ 6,053
Operating Income	145	691	325	220	140	626	237	256	1,381	1,259	1,254
Net income	33	371	148	107	30	345	151	110	659	636	592
Net income attributable to Ameren common shareholders - continuing operations	32	369	147	105	30	343	98	108	653	579	587
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations	0	0	0	0	(1)	0	52	0	0	51	(1)
Net income attributable to Ameren common shareholders	$ 32	$ 369	$ 147	$ 105	$ 29	$ 343	$ 150	$ 108	$ 653	$ 630	$ 586
Earnings per common share - basic - continuing operations	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 0.12	$ 1.42	$ 0.40	$ 0.45	$ 2.69	$ 2.39	$ 2.42
Earnings per common share - basic - discontinued operations	0.00	0.00	0.00	0.00	0.00	0.00	0.21	0.00	0.00	0.21	0.00
Earnings per common share - basic	0.13	1.52	0.61	0.43	0.12	1.42	0.61	0.45	2.69	2.60	2.42
Earnings per common share - diluted - continuing operations	0.13	1.52	0.61	0.43	0.12	1.41	0.40	0.45	2.68	2.38	2.40
Earnings per common share - diluted - discontinued operations	0.00	0.00	0.00	0.00	0.00	0.00	0.21	0.00	0.00	0.21	0.00
Earnings per common share - diluted	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 0.12	$ 1.41	$ 0.61	$ 0.45	$ 2.68	$ 2.59	$ 2.40
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 750	$ 1,165	$ 867	$ 741	$ 754	$ 1,171	$ 884	$ 800	$ 3,523	$ 3,609	$ 3,553
Operating Income	54	431	197	63	58	423	146	115	745	742	785
Net income	10	242	93	15	11	240	62	42	360	355	393
Net Income Available to Common Shareholder	10	241	92	14	11	239	61	41	357	352	390
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	595	676	542	677	553	655	513	745	2,490	2,466	2,498
Operating Income	74	230	107	133	74	189	83	120	544	466	450
Net income	30	119	46	60	33	98	32	54	255	217	204
Net Income Available to Common Shareholder	$ 29	$ 119	$ 45	$ 59	$ 32	$ 98	$ 31	$ 53	$ 252	$ 214	$ 201