Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 276
Unrecorded Unconditional Purchase Obligation, Due within Two Years	746
Unrecorded Unconditional Purchase Obligation, Due within Three Years	426
Unrecorded Unconditional Purchase Obligation, Due within Four Years	292
Unrecorded Unconditional Purchase Obligation, Due within Five Years	135
Unrecorded Unconditional Purchase Obligation, Due after Five Years	261
Unrecorded Unconditional Purchase Obligation	2,136
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	86
Unrecorded Unconditional Purchase Obligation, Due within Two Years	319
Unrecorded Unconditional Purchase Obligation, Due within Three Years	159
Unrecorded Unconditional Purchase Obligation, Due within Four Years	121
Unrecorded Unconditional Purchase Obligation, Due within Five Years	73
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	758
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	55
Unrecorded Unconditional Purchase Obligation, Due within Two Years	189
Unrecorded Unconditional Purchase Obligation, Due within Three Years	131
Unrecorded Unconditional Purchase Obligation, Due within Four Years	70
Unrecorded Unconditional Purchase Obligation, Due within Five Years	19
Unrecorded Unconditional Purchase Obligation, Due after Five Years	39
Unrecorded Unconditional Purchase Obligation	503
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	36
Unrecorded Unconditional Purchase Obligation, Due within Two Years	26
Unrecorded Unconditional Purchase Obligation, Due within Three Years	38
Unrecorded Unconditional Purchase Obligation, Due within Four Years	56
Unrecorded Unconditional Purchase Obligation, Due within Five Years	13
Unrecorded Unconditional Purchase Obligation, Due after Five Years	72
Unrecorded Unconditional Purchase Obligation	241
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	60
Unrecorded Unconditional Purchase Obligation, Due within Two Years	157
Unrecorded Unconditional Purchase Obligation, Due within Three Years	53
Unrecorded Unconditional Purchase Obligation, Due within Four Years	10
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 280
Amount of Megawatts | MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4
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	58
Unrecorded Unconditional Purchase Obligation	77
Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	38
Unrecorded Unconditional Purchase Obligation, Due within Two Years	51
Unrecorded Unconditional Purchase Obligation, Due within Three Years	41
Unrecorded Unconditional Purchase Obligation, Due within Four Years	30
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25
Unrecorded Unconditional Purchase Obligation, Due after Five Years	92
Unrecorded Unconditional Purchase Obligation	277
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	165
Unrecorded Unconditional Purchase Obligation, Due within Two Years	423
Unrecorded Unconditional Purchase Obligation, Due within Three Years	256
Unrecorded Unconditional Purchase Obligation, Due within Four Years	221
Unrecorded Unconditional Purchase Obligation, Due within Five Years	121
Unrecorded Unconditional Purchase Obligation, Due after Five Years	222
Unrecorded Unconditional Purchase Obligation	1,408
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	86
Unrecorded Unconditional Purchase Obligation, Due within Two Years	319
Unrecorded Unconditional Purchase Obligation, Due within Three Years	159
Unrecorded Unconditional Purchase Obligation, Due within Four Years	121
Unrecorded Unconditional Purchase Obligation, Due within Five Years	73
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	758
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	11
Unrecorded Unconditional Purchase Obligation, Due within Two Years	39
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
Unrecorded Unconditional Purchase Obligation, Due within Four Years	14
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	17
Unrecorded Unconditional Purchase Obligation	116
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	36
Unrecorded Unconditional Purchase Obligation, Due within Two Years	26
Unrecorded Unconditional Purchase Obligation, Due within Three Years	38
Unrecorded Unconditional Purchase Obligation, Due within Four Years	56
Unrecorded Unconditional Purchase Obligation, Due within Five Years	13
Unrecorded Unconditional Purchase Obligation, Due after Five Years	72
Unrecorded Unconditional Purchase Obligation	241
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	$ 1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4
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	58
Unrecorded Unconditional Purchase Obligation	77
Union Electric Company | Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	31
Unrecorded Unconditional Purchase Obligation, Due within Two Years	35
Unrecorded Unconditional Purchase Obligation, Due within Three Years	25
Unrecorded Unconditional Purchase Obligation, Due within Four Years	25
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25
Unrecorded Unconditional Purchase Obligation, Due after Five Years	75
Unrecorded Unconditional Purchase Obligation	216
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	108
Unrecorded Unconditional Purchase Obligation, Due within Two Years	314
Unrecorded Unconditional Purchase Obligation, Due within Three Years	160
Unrecorded Unconditional Purchase Obligation, Due within Four Years	66
Unrecorded Unconditional Purchase Obligation, Due within Five Years	14
Unrecorded Unconditional Purchase Obligation, Due after Five Years	22
Unrecorded Unconditional Purchase Obligation	684
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	44
Unrecorded Unconditional Purchase Obligation, Due within Two Years	150
Unrecorded Unconditional Purchase Obligation, Due within Three Years	100
Unrecorded Unconditional Purchase Obligation, Due within Four Years	56
Unrecorded Unconditional Purchase Obligation, Due within Five Years	14
Unrecorded Unconditional Purchase Obligation, Due after Five Years	22
Unrecorded Unconditional Purchase Obligation	386
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	60
Unrecorded Unconditional Purchase Obligation, Due within Two Years	157
Unrecorded Unconditional Purchase Obligation, Due within Three Years	53
Unrecorded Unconditional Purchase Obligation, Due within Four Years	10
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	280
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	4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	7
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	18
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 42
Ameren Illinois Company | April 2018 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours | MWh	3,956,200
Long-term Purchase Commitment, Amount	$ 112
Ameren Illinois Company | September 2018 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours | MWh	2,221,400
Long-term Purchase Commitment, Amount	$ 63
Ameren Illinois Company | September 2018 Procurement - Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatts | MWh	653
Long-term Purchase Commitment, Amount	$ 7