Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,659	$ 1,724	$ 4,594	$ 4,872
Operating Expenses:
Fuel	147	216	409	590
Purchased Power	148	148	440	453
Natural gas purchased for resale	31	30	236	252
Other operations and maintenance	434	429	1,301	1,299
Depreciation and amortization	248	241	745	713
Taxes other than income taxes	131	127	375	374
Total operating expenses	1,139	1,191	3,506	3,681
Operating Income	520	533	1,088	1,191
Other Income (Expense), Net	34	32	99	84
Interest Charges	96	101	290	302
Income Before Income Taxes	458	464	897	973
Income Taxes	92	105	158	221
Net income	366	359	739	752
Less: Net Income Attributable to Noncontrolling Interests	2	2	5	5
Net income attributable to Ameren common shareholders	364	357	734	747
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	366	361	740	753
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income	$ 364	$ 359	$ 735	$ 748
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 1.48	$ 1.46	$ 2.99	$ 3.06
Earnings Per Share, Diluted	$ 1.47	$ 1.45	$ 2.97	$ 3.04
Average Common Shares Outstanding - Basic (in shares)	245.9	244.1	245.5	243.6
Weighted Average Number of Shares Outstanding, Diluted (in shares)	247.5	246.3	247.0	245.5
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,528	$ 1,590	$ 3,928	$ 4,209
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	131	134	666	663
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,059	1,129	2,615	2,876
Operating Expenses:
Fuel	147	216	409	590
Purchased Power	49	49	160	131
Natural gas purchased for resale	6	5	41	37
Other operations and maintenance	242	234	720	707
Depreciation and amortization	138	137	417	411
Taxes other than income taxes	96	94	256	258
Total operating expenses	678	735	2,003	2,134
Operating Income	381	394	612	742
Other Income (Expense), Net	15	16	43	45
Interest Charges	44	50	136	152
Income Before Income Taxes	352	360	519	635
Income Taxes	51	65	70	132
Net income	301	295	449	503
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	300	294	446	500
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,040	1,111	2,517	2,782
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	19	18	98	94
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	564	564	1,873	1,902
Operating Expenses:
Purchased Power	101	105	284	334
Natural gas purchased for resale	25	25	195	215
Other operations and maintenance	193	195	580	590
Depreciation and amortization	102	94	304	278
Taxes other than income taxes	33	32	110	108
Total operating expenses	454	451	1,473	1,525
Operating Income	110	113	400	377
Other Income (Expense), Net	13	11	39	30
Interest Charges	38	38	111	112
Income Before Income Taxes	85	86	328	295
Income Taxes	20	23	79	73
Net income	65	63	249	222
Preferred Stock Dividends	0	0	2	2
Net Income (Loss) Attributable to Parent	65	63	247	220
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	452	448	1,305	1,333
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 112	$ 116	$ 568	$ 569