Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 600
Unrecorded Unconditional Purchase Obligation, Due within Two Years	460
Unrecorded Unconditional Purchase Obligation, Due within Three Years	307
Unrecorded Unconditional Purchase Obligation, Due within Four Years	146
Unrecorded Unconditional Purchase Obligation, Due within Five Years	81
Unrecorded Unconditional Purchase Obligation, Due after Five Years	161
Unrecorded Unconditional Purchase Obligation	1,755
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	279
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	121
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	632
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	154
Unrecorded Unconditional Purchase Obligation, Due within Two Years	158
Unrecorded Unconditional Purchase Obligation, Due within Three Years	85
Unrecorded Unconditional Purchase Obligation, Due within Four Years	32
Unrecorded Unconditional Purchase Obligation, Due within Five Years	8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	34
Unrecorded Unconditional Purchase Obligation	471
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	25
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	58
Unrecorded Unconditional Purchase Obligation, Due within Four Years	13
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	29
Unrecorded Unconditional Purchase Obligation	209
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	88
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10
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	$ 152
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	27
Unrecorded Unconditional Purchase Obligation	41
Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	52
Unrecorded Unconditional Purchase Obligation, Due within Two Years	42
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
Unrecorded Unconditional Purchase Obligation, Due within Four Years	26
Unrecorded Unconditional Purchase Obligation, Due within Five Years	29
Unrecorded Unconditional Purchase Obligation, Due after Five Years	71
Unrecorded Unconditional Purchase Obligation	250
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	378
Unrecorded Unconditional Purchase Obligation, Due within Two Years	269
Unrecorded Unconditional Purchase Obligation, Due within Three Years	223
Unrecorded Unconditional Purchase Obligation, Due within Four Years	119
Unrecorded Unconditional Purchase Obligation, Due within Five Years	78
Unrecorded Unconditional Purchase Obligation, Due after Five Years	124
Unrecorded Unconditional Purchase Obligation	1,191
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	279
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	121
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	632
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	33
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	106
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	25
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	58
Unrecorded Unconditional Purchase Obligation, Due within Four Years	13
Unrecorded Unconditional Purchase Obligation, Due within Five Years	41
Unrecorded Unconditional Purchase Obligation, Due after Five Years	29
Unrecorded Unconditional Purchase Obligation	209
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	27
Unrecorded Unconditional Purchase Obligation	41
Union Electric Company | Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	39
Unrecorded Unconditional Purchase Obligation, Due within Two Years	29
Unrecorded Unconditional Purchase Obligation, Due within Three Years	25
Unrecorded Unconditional Purchase Obligation, Due within Four Years	26
Unrecorded Unconditional Purchase Obligation, Due within Five Years	29
Unrecorded Unconditional Purchase Obligation, Due after Five Years	55
Unrecorded Unconditional Purchase Obligation	203
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	182
Unrecorded Unconditional Purchase Obligation, Due within Three Years	79
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	21
Unrecorded Unconditional Purchase Obligation	526
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	121
Unrecorded Unconditional Purchase Obligation, Due within Two Years	124
Unrecorded Unconditional Purchase Obligation, Due within Three Years	69
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	21
Unrecorded Unconditional Purchase Obligation	365
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	88
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10
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	152
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	5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
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	9
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 10
Ameren Illinois Company | April 2019 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours | MWh	3,704,000
Long-term Purchase Commitment, Amount	$ 108