Commitments And Contingencies (Schedule Of Estimated Purchased Power Commitments) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	$ 981
2018	676
2019	288
2020	151
2021	55
Thereafter	254
Total	2,405
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	463
2018	383
2019	85
2020	27
2021	0
Thereafter	0
Total	958
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	205
2018	163
2019	110
2020	46
2021	11
Thereafter	38
Total	573
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	67
2018	26
2019	39
2020	45
2021	12
Thereafter	45
Total	234
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	170
2018	63
2019	14
2020	3
2021	2
Thereafter	18
Total	270
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	3
2018	4
2019	4
2020	5
2021	5
Thereafter	58
Total	79
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	73
2018	37
2019	36
2020	25
2021	25
Thereafter	95
Total	291
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	628
2018	473
2019	181
2020	115
2021	48
Thereafter	194
Total	1,639
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	463
2018	383
2019	85
2020	27
2021	0
Thereafter	0
Total	958
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	42
2018	36
2019	29
2020	13
2021	6
Thereafter	16
Total	142
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	67
2018	26
2019	39
2020	45
2021	12
Thereafter	45
Total	234
Union Electric Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	0
2018	0
2019	0
2020	0
2021	0
Thereafter	0
Total	0
Union Electric Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	3
2018	4
2019	4
2020	5
2021	5
Thereafter	58
Total	79
Union Electric Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	53
2018	24
2019	24
2020	25
2021	25
Thereafter	75
Total	226
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	352
2018	203
2019	107
2020	36
2021	7
Thereafter	40
Total	745
Ameren Illinois Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	163
2018	127
2019	81
2020	33
2021	5
Thereafter	22
Total	431
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	170
2018	63
2019	14
2020	3
2021	2
Thereafter	18
Total	270
Ameren Illinois Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	19
2018	13
2019	12
2020	0
2021	0
Thereafter	0
Total	$ 44