Consolidated Statement of Income (Loss) - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating Revenues:
Electric		$ 1,594	$ 1,725	$ 4,183	$ 4,101
Gas		129	134	592	619
Total operating revenues		1,723	1,859	4,775	4,720
Operating Expenses:
Fuel		199	205	594	574
Purchased power		162	178	491	451
Gas purchased for resale		25	34	196	227
Other operations and maintenance		402	411	1,229	1,246
Depreciation and amortization		225	211	668	628
Taxes other than income taxes		129	129	364	358
Total operating expenses		1,142	1,168	3,542	3,484
Operating Income		581	691	1,233	1,236
Other Income and Expenses:
Miscellaneous income	[1]	13	18	42	54
Miscellaneous expense	[1]	2	8	16	21
Total other income (expense)		11	10	26	33
Interest Charges		97	97	295	287
Income Before Income Taxes		495	604	964	982
Income Taxes		205	233	376	356
Net income		290	371	588	626
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)	2	1
Comprehensive Income		288	368	585	622
Less: Net Income Attributable to Noncontrolling Interests		2	2	5	5
Net Income Attributable to Ameren Common Shareholders		$ 288	$ 369	$ 583	$ 621
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic		$ 1.19	$ 1.52	$ 2.40	$ 2.56
Earnings Per Share, Diluted		1.18	1.52	2.39	2.56
Dividends per Common Share		$ 0.44	$ 0.425	$ 1.32	$ 1.275
Average Common Shares Outstanding - Basic		242.6	242.6	242.6	242.6
Average Common Shares Outstanding - Diluted		244.7	242.9	244.0	243.0
Union Electric Company
Operating Revenues:
Electric		$ 1,098	$ 1,144	$ 2,757	$ 2,682
Gas		17	20	83	90
Other Revenue, Net		0	1	0	1
Total operating revenues		1,115	1,165	2,840	2,773
Operating Expenses:
Fuel		199	205	594	574
Purchased power		42	77	201	169
Gas purchased for resale		4	6	29	33
Other operations and maintenance		224	220	655	670
Depreciation and amortization		134	130	399	384
Taxes other than income taxes		95	96	255	252
Total operating expenses		698	734	2,133	2,082
Operating Income		417	431	707	691
Other Income and Expenses:
Miscellaneous income		13	14	36	38
Miscellaneous expense		2	2	6	6
Total other income (expense)		11	12	30	32
Interest Charges		50	53	157	158
Income Before Income Taxes		378	390	580	565
Income Taxes		143	148	218	215
Net income		235	242	362	350
Other Comprehensive Income		0	0	0	0
Comprehensive Income		235	242	362	350
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1	3	3
Net Income Available to Common Stockholder		234	241	359	347
Ameren Illinois Company
Operating Revenues:
Electric		463	562	1,343	1,365
Gas		112	114	510	530
Other Revenue, Net		0	0	1	0
Total operating revenues		575	676	1,854	1,895
Operating Expenses:
Purchased power		124	110	312	304
Gas purchased for resale		21	28	167	194
Other operations and maintenance		183	198	590	592
Depreciation and amortization		86	80	254	237
Taxes other than income taxes		33	30	101	98
Total operating expenses		447	446	1,424	1,425
Operating Income		128	230	430	470
Other Income and Expenses:
Miscellaneous income		1	4	7	15
Miscellaneous expense		0	3	8	11
Total other income (expense)		1	1	(1)	4
Interest Charges		36	35	109	105
Income Before Income Taxes		93	196	320	369
Income Taxes		38	77	127	144
Net income		55	119	193	225
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)	0	(3)
Comprehensive Income		55	118	193	222
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		0	0	2	2
Net Income Available to Common Stockholder		$ 55	$ 119	$ 191	$ 223

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.