Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,379	$ 1,563	$ 2,935	$ 3,148
Operating Expenses:
Fuel	102	186	262	374
Purchased Power	136	142	292	305
Natural gas purchased for resale	44	51	205	222
Other operations and maintenance	450	439	867	870
Depreciation and amortization	249	238	497	472
Taxes other than income taxes	118	122	244	247
Total operating expenses	1,099	1,178	2,367	2,490
Operating Income	280	385	568	658
Other Income (Expense), Net	36	29	65	52
Interest Charges	97	100	194	201
Income Before Income Taxes	219	314	439	509
Income Taxes	39	74	66	116
Net income	180	240	373	393
Less: Net Income Attributable to Noncontrolling Interests	1	1	3	3
Net Income (Loss) Available to Common Stockholders, Basic	179	239	370	390
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	(2)	1	(1)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	180	238	374	392
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	1	1	3	3
Comprehensive Income	$ 179	$ 237	$ 371	$ 389
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 0.73	$ 0.98	$ 1.51	$ 1.60
Earnings Per Share, Diluted	$ 0.72	$ 0.97	$ 1.50	$ 1.59
Average Common Shares Outstanding - Basic (in shares)	245.6	243.7	245.3	243.3
Weighted Average Number of Shares Outstanding, Diluted (in shares)	247.2	245.8	246.8	245.1
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,218	$ 1,396	$ 2,400	$ 2,619
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	161	167	535	529
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	798	955	1,556	1,747
Operating Expenses:
Fuel	102	186	262	374
Purchased Power	60	40	111	82
Natural gas purchased for resale	8	8	35	32
Other operations and maintenance	254	241	478	473
Depreciation and amortization	139	138	279	274
Taxes other than income taxes	83	84	160	164
Total operating expenses	646	697	1,325	1,399
Operating Income	152	258	231	348
Other Income (Expense), Net	16	16	28	29
Interest Charges	45	51	92	102
Income Before Income Taxes	123	223	167	275
Income Taxes	15	54	19	67
Net income	108	169	148	208
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	107	168	146	206
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	773	930	1,477	1,671
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	25	25	79	76
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	547	578	1,309	1,338
Operating Expenses:
Purchased Power	78	105	183	229
Natural gas purchased for resale	36	43	170	190
Other operations and maintenance	196	196	387	395
Depreciation and amortization	101	94	202	184
Taxes other than income taxes	32	35	77	76
Total operating expenses	443	473	1,019	1,074
Operating Income	104	105	290	264
Other Income (Expense), Net	15	13	26	19
Interest Charges	36	37	73	74
Income Before Income Taxes	83	81	243	209
Income Taxes	20	18	59	50
Net income	63	63	184	159
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	62	62	182	157
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	411	436	853	885
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 136	$ 142	$ 456	$ 453