Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 568
Unrecorded Unconditional Purchase Obligation, Due within Two Years	656
Unrecorded Unconditional Purchase Obligation, Due within Three Years	344
Unrecorded Unconditional Purchase Obligation, Due within Four Years	183
Unrecorded Unconditional Purchase Obligation, Due within Five Years	55
Unrecorded Unconditional Purchase Obligation, Due after Five Years	252
Unrecorded Unconditional Purchase Obligation	2,058
Coal [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	181
Unrecorded Unconditional Purchase Obligation, Due within Two Years	246
Unrecorded Unconditional Purchase Obligation, Due within Three Years	85
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	539
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	148
Unrecorded Unconditional Purchase Obligation, Due within Two Years	210
Unrecorded Unconditional Purchase Obligation, Due within Three Years	125
Unrecorded Unconditional Purchase Obligation, Due within Four Years	61
Unrecorded Unconditional Purchase Obligation, Due within Five Years	12
Unrecorded Unconditional Purchase Obligation, Due after Five Years	39
Unrecorded Unconditional Purchase Obligation	595
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	51
Unrecorded Unconditional Purchase Obligation, Due within Two Years	27
Unrecorded Unconditional Purchase Obligation, Due within Three Years	38
Unrecorded Unconditional Purchase Obligation, Due within Four Years	57
Unrecorded Unconditional Purchase Obligation, Due within Five Years	12
Unrecorded Unconditional Purchase Obligation, Due after Five Years	62
Unrecorded Unconditional Purchase Obligation	247
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	130
Unrecorded Unconditional Purchase Obligation, Due within Two Years	122
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
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	287
Methane Gas [Member]
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	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	78
Other Purchased Fuel [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	56
Unrecorded Unconditional Purchase Obligation, Due within Two Years	47
Unrecorded Unconditional Purchase Obligation, Due within Three Years	62
Unrecorded Unconditional Purchase Obligation, Due within Four Years	28
Unrecorded Unconditional Purchase Obligation, Due within Five Years	26
Unrecorded Unconditional Purchase Obligation, Due after Five Years	93
Unrecorded Unconditional Purchase Obligation	312
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	295
Unrecorded Unconditional Purchase Obligation, Due within Two Years	344
Unrecorded Unconditional Purchase Obligation, Due within Three Years	201
Unrecorded Unconditional Purchase Obligation, Due within Four Years	128
Unrecorded Unconditional Purchase Obligation, Due within Five Years	48
Unrecorded Unconditional Purchase Obligation, Due after Five Years	211
Unrecorded Unconditional Purchase Obligation	1,227
Union Electric Company | Coal [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	181
Unrecorded Unconditional Purchase Obligation, Due within Two Years	246
Unrecorded Unconditional Purchase Obligation, Due within Three Years	85
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	539
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	22
Unrecorded Unconditional Purchase Obligation, Due within Two Years	38
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	126
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	51
Unrecorded Unconditional Purchase Obligation, Due within Two Years	27
Unrecorded Unconditional Purchase Obligation, Due within Three Years	38
Unrecorded Unconditional Purchase Obligation, Due within Four Years	57
Unrecorded Unconditional Purchase Obligation, Due within Five Years	12
Unrecorded Unconditional Purchase Obligation, Due after Five Years	62
Unrecorded Unconditional Purchase Obligation	247
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 [Member]
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	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	78
Union Electric Company | Other Purchased Fuel [Member]
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	44
Unrecorded Unconditional Purchase Obligation, Due within Four Years	25
Unrecorded Unconditional Purchase Obligation, Due within Five Years	26
Unrecorded Unconditional Purchase Obligation, Due after Five Years	74
Unrecorded Unconditional Purchase Obligation	237
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	261
Unrecorded Unconditional Purchase Obligation, Due within Two Years	303
Unrecorded Unconditional Purchase Obligation, Due within Three Years	134
Unrecorded Unconditional Purchase Obligation, Due within Four Years	52
Unrecorded Unconditional Purchase Obligation, Due within Five Years	7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	22
Unrecorded Unconditional Purchase Obligation	779
Ameren Illinois Company | Coal [Member]
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	126
Unrecorded Unconditional Purchase Obligation, Due within Two Years	172
Unrecorded Unconditional Purchase Obligation, Due within Three Years	95
Unrecorded Unconditional Purchase Obligation, Due within Four Years	47
Unrecorded Unconditional Purchase Obligation, Due within Five Years	7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	22
Unrecorded Unconditional Purchase Obligation	469
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	130
Unrecorded Unconditional Purchase Obligation, Due within Two Years	122
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
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	287
Ameren Illinois Company | Methane Gas [Member]
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 [Member]
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	9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	9
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	23
Ameren Illinois Company | April 2018 Procurement [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Long-term Purchase Commitment, Amount	$ 112
Amount of Megawatthours | MWh	3,956,200
Ameren Illinois Company | Zero Emission Credits [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 57