Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,556	$ 1,585
Operating Expenses:
Fuel	160	188
Utilities Operating Expense, Purchased Power	156	163
Utilities Operating Expense, Gas and Petroleum Purchased	161	171
Other operations and maintenance	417	431
Depreciation and amortization	248	234
Taxes other than income taxes	126	125
Total operating expenses	1,268	1,312
Operating Income	288	273
Other Income (Expense), Net	29	23
Interest Charges	97	101
Income Before Income Taxes	220	195
Income Taxes	27	42
Net income	193	153
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net Income (Loss) Available to Common Stockholders, Basic	191	151
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	194	154
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income	$ 192	$ 152
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted	$ 0.78	$ 0.62
Average Common Shares Outstanding - Basic (in shares)	244.9	242.9
Weighted Average Number of Shares Outstanding, Diluted	246.4	244.4
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,182	$ 1,223
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	374	362
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	758	792
Operating Expenses:
Fuel	160	188
Utilities Operating Expense, Purchased Power	51	42
Utilities Operating Expense, Gas and Petroleum Purchased	27	24
Other operations and maintenance	224	232
Depreciation and amortization	140	136
Taxes other than income taxes	77	80
Total operating expenses	679	702
Operating Income	79	90
Other Income (Expense), Net	12	13
Interest Charges	47	51
Income Before Income Taxes	44	52
Income Taxes	4	13
Net income	40	39
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	39	38
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	704	741
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	54	51
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	762	760
Operating Expenses:
Utilities Operating Expense, Purchased Power	105	124
Utilities Operating Expense, Gas and Petroleum Purchased	134	147
Other operations and maintenance	191	199
Depreciation and amortization	101	90
Taxes other than income taxes	45	41
Total operating expenses	576	601
Operating Income	186	159
Other Income (Expense), Net	11	6
Interest Charges	37	37
Income Before Income Taxes	160	128
Income Taxes	39	32
Net income	121	96
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	120	95
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	442	449
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 320	$ 311