Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 1,370	$ 1,670	$ 1,419	$ 1,594	$ 6,098	$ 6,053	$ 5,838
Operating Income	140	626	237	256	125	561	322	246	1,259	1,254	1,184
Net income	30	345	151	110	49	295	150	98	636	592	295
Net income attributable to Ameren common shareholders - continuing operations	30	343	98	108	46	294	150	97	579	587	512
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations	(1)	0	52	0	2	(1)	(1)	(1)	51	(1)	(223)
Net income attributable to Ameren common shareholders	$ 29	$ 343	$ 150	$ 108	$ 48	$ 293	$ 149	$ 96	$ 630	$ 586	$ 289
Earnings per common share - basic - continuing operations	$ 0.12	$ 1.42	$ 0.40	$ 0.45	$ 0.19	$ 1.21	$ 0.62	$ 0.40	$ 2.39	$ 2.42	$ 2.11
Earnings (loss) per common share - basic - discontinued operations	0.00	0.00	0.21	0.00	0.01	0.00	(0.01)	0.00	0.21	0.00	(0.92)
Earnings per common share - basic	0.12	1.42	0.61	0.45	0.20	1.21	0.61	0.40	2.60	2.42	1.19
Earnings per common share - diluted - continuing operations	0.12	1.41	0.40	0.45	0.19	1.20	0.62	0.40	2.38	2.40	2.10
Earnings (loss) per common share - diluted - discontinued operations	0.00	0.00	0.21	0.00	0.01	0.00	(0.01)	0.00	0.21	0.00	(0.92)
Earnings per common share - diluted	$ 0.12	$ 1.41	$ 0.61	$ 0.45	$ 0.20	$ 1.20	$ 0.61	$ 0.40	$ 2.59	$ 2.40	$ 1.18
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 754	$ 1,171	$ 884	$ 800	$ 739	$ 1,097	$ 900	$ 817	$ 3,609	$ 3,553	$ 3,541
Operating Income	58	423	146	115	29	394	243	119	742	785	803
Net income									355	393	398
Net income attributable to Ameren common shareholders	11	240	62	42	(5)	223	127	48	355	393	398
Net Income (Loss) Available to Common Shareholder	11	239	61	41	(5)	222	126	47	352	390	395
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	553	655	513	745	633	572	519	774	2,466	2,498	2,311
Operating Income	74	189	83	120	97	158	75	120	466	450	415
Net income									217	204	163
Net income attributable to Ameren common shareholders	33	98	32	54	46	75	29	54	217	204	163
Net Income (Loss) Available to Common Shareholder	$ 32	$ 98	$ 31	$ 53	$ 45	$ 75	$ 28	$ 53	$ 214	$ 201	$ 160