Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,398	$ 1,379	$ 2,838	$ 2,935
Operating Expenses:
Fuel	119	102	259	262
Purchased Power	109	136	243	292
Natural gas purchased for resale	42	44	149	205
Other operations and maintenance	384	450	822	867
Depreciation and amortization	271	249	526	497
Taxes other than income taxes	119	118	244	244
Total operating expenses	1,044	1,099	2,243	2,367
Operating Income	354	280	595	568
Other Income (Expense), Net	48	36	69	65
Interest Charges	108	97	201	194
Income Before Income Taxes	294	219	463	439
Income Taxes	50	39	71	66
Net income	244	180	392	373
Less: Net Income Attributable to Noncontrolling Interests	1	1	3	3
Net income attributable to Ameren common shareholders	243	179	389	370
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	0	1	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	244	180	393	374
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	1	1	3	3
Comprehensive Income	243	179	390	371
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	$ 0	$ 0	$ 0	$ 0
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 0.99	$ 0.73	$ 1.58	$ 1.51
Earnings Per Share, Diluted	$ 0.98	$ 0.72	$ 1.57	$ 1.50
Average Common Shares Outstanding - Basic (in shares)	246.9	245.6	246.7	245.3
Weighted Average Number of Shares Outstanding, Diluted (in shares)	247.9	247.2	248.0	246.8
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,237	$ 1,218	$ 2,357	$ 2,400
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	161	161	481	535
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	792	798	1,472	1,556
Operating Expenses:
Fuel	119	102	259	262
Purchased Power	37	60	76	111
Natural gas purchased for resale	6	8	24	35
Other operations and maintenance	202	254	441	478
Depreciation and amortization	155	139	294	279
Taxes other than income taxes	83	83	162	160
Total operating expenses	602	646	1,256	1,325
Operating Income	190	152	216	231
Other Income (Expense), Net	25	16	29	28
Interest Charges	50	45	90	92
Income Before Income Taxes	165	123	155	167
Income Taxes	12	15	11	19
Net income	153	108	144	148
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	152	107	142	146
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	771	773	1,402	1,477
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	21	25	70	79
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	567	547	1,290	1,309
Operating Expenses:
Purchased Power	76	78	174	183
Natural gas purchased for resale	36	36	125	170
Other operations and maintenance	182	196	381	387
Depreciation and amortization	107	101	214	202
Taxes other than income taxes	32	32	74	77
Total operating expenses	433	443	968	1,019
Operating Income	134	104	322	290
Other Income (Expense), Net	17	15	28	26
Interest Charges	38	36	77	73
Income Before Income Taxes	113	83	273	243
Income Taxes	29	20	68	59
Net income	84	63	205	184
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	83	62	203	182
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	427	411	879	853
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 140	$ 136	$ 411	$ 456