Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,440	$ 1,556
Operating Expenses:
Fuel	140	160
Purchased Power	134	156
Natural gas purchased for resale	107	161
Other operations and maintenance	438	417
Depreciation and amortization	255	248
Taxes other than income taxes	125	126
Total operating expenses	1,199	1,268
Operating Income	241	288
Other Income (Expense), Net	21	29
Interest Charges	93	97
Income Before Income Taxes	169	220
Income Taxes	21	27
Net income	148	193
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net income attributable to Ameren common shareholders	146	191
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	149	194
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income	$ 147	$ 192
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted	$ 0.59	$ 0.78
Average Common Shares Outstanding - Basic (in shares)	246.4	244.9
Weighted Average Number of Shares Outstanding, Diluted (in shares)	248.1	246.4
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,120	$ 1,182
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	320	374
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	680	758
Operating Expenses:
Fuel	140	160
Purchased Power	39	51
Natural gas purchased for resale	18	27
Other operations and maintenance	239	224
Depreciation and amortization	139	140
Taxes other than income taxes	79	77
Total operating expenses	654	679
Operating Income	26	79
Other Income (Expense), Net	4	12
Interest Charges	40	47
Income Before Income Taxes	(10)	44
Income Taxes	(1)	4
Net income	(9)	40
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	(10)	39
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	631	704
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	49	54
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	723	762
Operating Expenses:
Purchased Power	98	105
Natural gas purchased for resale	89	134
Other operations and maintenance	199	191
Depreciation and amortization	107	101
Taxes other than income taxes	42	45
Total operating expenses	535	576
Operating Income	188	186
Other Income (Expense), Net	11	11
Interest Charges	39	37
Income Before Income Taxes	160	160
Income Taxes	39	39
Net income	121	121
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	120	120
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	452	442
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 271	$ 320