Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									$ 511	$ 299	$ 299
Operating revenues	$ 1,402	$ 1,723	$ 1,538	$ 1,514	$ 1,356	$ 1,859	$ 1,427	$ 1,434	6,177	6,076	6,098
Operating Income	225	581	398	254	145	691	325	220	1,458	1,381	1,259
Net income	(59)	290	194	104	33	371	148	107	529	659	636
Net income attributable to Ameren common shareholders	$ (60)	$ 288	$ 193	$ 102	$ 32	$ 369	$ 147	$ 105	$ 523	$ 653	$ 630
Earnings per common share - basic (in dollars per share)	$ (0.24)	$ 1.19	$ 0.79	$ 0.42	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 2.16	$ 2.69	$ 2.60
Earnings per common share - diluted (in dollars per share)	$ (0.24)	$ 1.18	$ 0.79	$ 0.42	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 2.14	$ 2.68	$ 2.59
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									$ 76	$ 161	$ 71
Operating revenues	$ 699	$ 1,115	$ 935	$ 790	$ 750	$ 1,165	$ 867	$ 741	3,539	3,523	3,609
Operating Income	40	417	237	53	54	431	197	63	747	745	742
Net income	(36)	235	121	6	10	242	93	15	326	360	355
Net Income Available to Common Shareholder	(36)	234	120	5	10	241	92	14	323	357	352
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									185	117	193
Operating revenues	674	575	576	703	595	676	542	677	2,528	2,490	2,466
Operating Income	150	128	130	172	74	230	107	133	580	544	466
Net income	78	55	58	80	30	119	46	60	271	255	217
Net Income Available to Common Shareholder	$ 77	$ 55	$ 57	$ 79	$ 29	$ 119	$ 45	$ 59	268	$ 252	$ 214
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)