Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,563	$ 1,537	$ 3,148	$ 3,052
Operating Expenses:
Fuel	186	189	374	395
Utilities Operating Expense, Purchased Power	142	150	305	330
Utilities Operating Expense, Gas and Petroleum Purchased	51	41	222	171
Other operations and maintenance	439	431	870	849
Depreciation and amortization	238	222	472	443
Taxes other than income taxes	122	117	247	235
Total operating expenses	1,178	1,150	2,490	2,423
Operating Income	385	387	658	629
Other Income (Expense), Net	29	20	52	38
Interest Charges	100	99	201	198
Income Before Income Taxes	314	308	509	469
Income Taxes	74	114	116	171
Net income	240	194	393	298
Less: Net Income Attributable to Noncontrolling Interests	1	1	3	3
Net Income (Loss) Available to Common Stockholders, Basic	239	193	390	295
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	(2)	2	(1)	2
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	238	196	392	300
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	1	1	3	3
Comprehensive Income	$ 237	$ 195	$ 389	$ 297
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 0.98	$ 0.79	$ 1.60	$ 1.21
Earnings Per Share, Diluted	0.97	0.79	1.59	1.21
Dividends per Common Share (in dollars per share)	$ 0.4575	$ 0.44	$ 0.915	$ 0.88
Average Common Shares Outstanding - Basic (in shares)	243.7	242.6	243.3	242.6
Weighted Average Number of Shares Outstanding, Diluted	245.8	243.5	245.1	243.7
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,396	$ 1,382	$ 2,619	$ 2,589
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	167	155	529	463
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	955	934	1,747	1,725
Operating Expenses:
Fuel	186	189	374	395
Utilities Operating Expense, Purchased Power	40	69	82	160
Utilities Operating Expense, Gas and Petroleum Purchased	8	5	32	25
Other operations and maintenance	241	224	473	443
Depreciation and amortization	138	132	274	265
Taxes other than income taxes	84	85	164	160
Total operating expenses	697	704	1,399	1,448
Operating Income	258	230	348	277
Other Income (Expense), Net	16	16	29	32
Interest Charges	51	53	102	107
Income Before Income Taxes	223	193	275	202
Income Taxes	54	72	67	75
Net income	169	121	208	127
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	168	120	206	125
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	930	912	1,671	1,659
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	25	22	76	66
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	578	576	1,338	1,279
Operating Expenses:
Utilities Operating Expense, Purchased Power	105	87	229	188
Utilities Operating Expense, Gas and Petroleum Purchased	43	36	190	146
Other operations and maintenance	196	212	395	412
Depreciation and amortization	94	85	184	168
Taxes other than income taxes	35	28	76	68
Total operating expenses	473	448	1,074	982
Operating Income	105	128	264	297
Other Income (Expense), Net	13	3	19	3
Interest Charges	37	36	74	73
Income Before Income Taxes	81	95	209	227
Income Taxes	18	37	50	89
Net income	63	58	159	138
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	62	57	157	136
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	436	442	885	881
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 142	$ 134	$ 453	$ 398