Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 516
Unrecorded Unconditional Purchase Obligation, Due within Two Years	560
Unrecorded Unconditional Purchase Obligation, Due within Three Years	393
Unrecorded Unconditional Purchase Obligation, Due within Four Years	199
Unrecorded Unconditional Purchase Obligation, Due within Five Years	128
Unrecorded Unconditional Purchase Obligation, Due after Five Years	143
Unrecorded Unconditional Purchase Obligation	1,939
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	224
Unrecorded Unconditional Purchase Obligation, Due within Two Years	207
Unrecorded Unconditional Purchase Obligation, Due within Three Years	183
Unrecorded Unconditional Purchase Obligation, Due within Four Years	125
Unrecorded Unconditional Purchase Obligation, Due within Five Years	46
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	785
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	113
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	96
Unrecorded Unconditional Purchase Obligation, Due within Four Years	38
Unrecorded Unconditional Purchase Obligation, Due within Five Years	20
Unrecorded Unconditional Purchase Obligation, Due after Five Years	36
Unrecorded Unconditional Purchase Obligation	463
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	23
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	60
Unrecorded Unconditional Purchase Obligation, Due within Four Years	11
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	28
Unrecorded Unconditional Purchase Obligation	206
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	127
Unrecorded Unconditional Purchase Obligation, Due within Two Years	114
Unrecorded Unconditional Purchase Obligation, Due within Three Years	29
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 275
Amount of Megawatts | MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	26
Unrecorded Unconditional Purchase Obligation	40
Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	27
Unrecorded Unconditional Purchase Obligation, Due within Two Years	33
Unrecorded Unconditional Purchase Obligation, Due within Three Years	22
Unrecorded Unconditional Purchase Obligation, Due within Four Years	17
Unrecorded Unconditional Purchase Obligation, Due within Five Years	18
Unrecorded Unconditional Purchase Obligation, Due after Five Years	53
Unrecorded Unconditional Purchase Obligation	170
Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	5
Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	60
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	297
Unrecorded Unconditional Purchase Obligation, Due within Two Years	308
Unrecorded Unconditional Purchase Obligation, Due within Three Years	279
Unrecorded Unconditional Purchase Obligation, Due within Four Years	161
Unrecorded Unconditional Purchase Obligation, Due within Five Years	113
Unrecorded Unconditional Purchase Obligation, Due after Five Years	106
Unrecorded Unconditional Purchase Obligation	1,264
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	224
Unrecorded Unconditional Purchase Obligation, Due within Two Years	207
Unrecorded Unconditional Purchase Obligation, Due within Three Years	183
Unrecorded Unconditional Purchase Obligation, Due within Four Years	125
Unrecorded Unconditional Purchase Obligation, Due within Five Years	46
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	785
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	28
Unrecorded Unconditional Purchase Obligation, Due within Two Years	34
Unrecorded Unconditional Purchase Obligation, Due within Three Years	16
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	13
Unrecorded Unconditional Purchase Obligation	101
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	23
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	60
Unrecorded Unconditional Purchase Obligation, Due within Four Years	11
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	28
Unrecorded Unconditional Purchase Obligation	206
Union Electric Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 0
Amount of Megawatts | MWh	102
Union Electric Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	26
Unrecorded Unconditional Purchase Obligation	40
Union Electric Company | Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	20
Unrecorded Unconditional Purchase Obligation, Due within Two Years	21
Unrecorded Unconditional Purchase Obligation, Due within Three Years	17
Unrecorded Unconditional Purchase Obligation, Due within Four Years	17
Unrecorded Unconditional Purchase Obligation, Due within Five Years	18
Unrecorded Unconditional Purchase Obligation, Due after Five Years	39
Unrecorded Unconditional Purchase Obligation	132
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	214
Unrecorded Unconditional Purchase Obligation, Due within Two Years	243
Unrecorded Unconditional Purchase Obligation, Due within Three Years	109
Unrecorded Unconditional Purchase Obligation, Due within Four Years	38
Unrecorded Unconditional Purchase Obligation, Due within Five Years	15
Unrecorded Unconditional Purchase Obligation, Due after Five Years	23
Unrecorded Unconditional Purchase Obligation	642
Ameren Illinois Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	0
Ameren Illinois Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	85
Unrecorded Unconditional Purchase Obligation, Due within Two Years	126
Unrecorded Unconditional Purchase Obligation, Due within Three Years	80
Unrecorded Unconditional Purchase Obligation, Due within Four Years	33
Unrecorded Unconditional Purchase Obligation, Due within Five Years	15
Unrecorded Unconditional Purchase Obligation, Due after Five Years	23
Unrecorded Unconditional Purchase Obligation	362
Ameren Illinois Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	0
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	127
Unrecorded Unconditional Purchase Obligation, Due within Two Years	114
Unrecorded Unconditional Purchase Obligation, Due within Three Years	29
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	275
Ameren Illinois Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	0
Ameren Illinois Company | Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	5
Ameren Illinois Company | Renewable Energy Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	5
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 60