Consolidated Statement of Income (Loss) - USD ($) shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating Revenues:
Electric		$ 1,383	$ 1,274	$ 2,589	$ 2,376
Gas		155	153	463	485
Total operating revenues		1,538	1,427	3,052	2,861
Operating Expenses:
Fuel		189	166	395	369
Purchased power		149	135	329	273
Gas purchased for resale		41	41	171	193
Other operations and maintenance		422	435	827	835
Depreciation and amortization		222	210	443	417
Taxes other than income taxes		117	115	235	229
Total operating expenses		1,140	1,102	2,400	2,316
Operating Income		398	325	652	545
Other Income and Expenses:
Miscellaneous income	[1]	14	16	29	36
Miscellaneous expense	[1]	5	6	14	13
Total other income (expense)		9	10	15	23
Interest Charges		99	95	198	190
Income Before Income Taxes		308	240	469	378
Income Taxes		114	92	171	123
Net income		194	148	298	255
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		2	4	2	2
Comprehensive Income		195	151	297	254
Less: Net Income Attributable to Noncontrolling Interests		1	1	3	3
Net Income Attributable to Ameren Common Shareholders		$ 193	$ 147	$ 295	$ 252
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted		$ 0.79	$ 0.61	$ 1.21	$ 1.04
Dividends per Common Share		$ 0.44	$ 0.425	$ 0.88	$ 0.85
Average Common Shares Outstanding - Basic		242.6	242.6	242.6	242.6
Union Electric Company
Operating Revenues:
Electric		$ 913	$ 844	$ 1,659	$ 1,538
Gas		22	23	66	70
Total operating revenues		935	867	1,725	1,608
Operating Expenses:
Fuel		189	166	395	369
Purchased power		68	50	159	92
Gas purchased for resale		5	6	25	27
Other operations and maintenance		219	238	431	450
Depreciation and amortization		132	127	265	254
Taxes other than income taxes		85	83	160	156
Total operating expenses		698	670	1,435	1,348
Operating Income		237	197	290	260
Other Income and Expenses:
Miscellaneous income		11	9	23	24
Miscellaneous expense		2	2	4	4
Total other income (expense)		9	7	19	20
Interest Charges		53	53	107	105
Income Before Income Taxes		193	151	202	175
Income Taxes		72	58	75	67
Net income		121	93	127	108
Other Comprehensive Income		0	0	0	0
Comprehensive Income		121	93	127	108
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1	2	2
Net Income Available to Common Stockholder		120	92	125	106
Ameren Illinois Company
Operating Revenues:
Electric		441	411	880	803
Gas		134	131	398	416
Other Revenue, Net		1	0	1	0
Total operating revenues		576	542	1,279	1,219
Operating Expenses:
Purchased power		87	90	188	194
Gas purchased for resale		36	35	146	166
Other operations and maintenance		210	200	407	394
Depreciation and amortization		85	80	168	157
Taxes other than income taxes		28	30	68	68
Total operating expenses		446	435	977	979
Operating Income		130	107	302	240
Other Income and Expenses:
Miscellaneous income		3	6	6	11
Miscellaneous expense		2	3	8	8
Total other income (expense)		1	3	(2)	3
Interest Charges		36	35	73	70
Income Before Income Taxes		95	75	227	173
Income Taxes		37	29	89	67
Net income		58	46	138	106
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)	0	(2)
Comprehensive Income		58	45	138	104
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1	2	2
Net Income Available to Common Stockholder		$ 57	$ 45	$ 136	$ 104

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