Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) MWh
Long-term Purchase Commitment [Line Items]
2017	$ 339	[1]
2018	951	[1]
2019	637	[1]
2020	269	[1]
2021	141	[1]
Thereafter	312	[1]
Total	2,649	[1]
Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	216
2018	610
2019	446
2020	174
2021	116
Thereafter	244
Total	1,806
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2017	123
2018	341
2019	191
2020	95
2021	25
Thereafter	45
Total	820
Coal
Long-term Purchase Commitment [Line Items]
2017	162	[1]
2018	453	[1]
2019	356	[1]
2020	79	[1]
2021	27	[1]
Thereafter	0	[1]
Total	1,077	[1]
Coal | Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	162
2018	453
2019	356
2020	79
2021	27
Thereafter	0
Total	1,077
Natural Gas
Long-term Purchase Commitment [Line Items]
2017	65	[1],[2]
2018	200	[1],[2]
2019	148	[1],[2]
2020	94	[1],[2]
2021	36	[1],[2]
Thereafter	47	[1],[2]
Total	590	[1],[2]
Natural Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	14	[2]
2018	42	[2]
2019	34	[2]
2020	26	[2]
2021	13	[2]
Thereafter	22	[2]
Total	151	[2]
Natural Gas | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2017	51	[2]
2018	158	[2]
2019	114	[2]
2020	68	[2]
2021	23	[2]
Thereafter	25	[2]
Total	439	[2]
Nuclear Fuel
Long-term Purchase Commitment [Line Items]
2017	19	[1]
2018	67	[1]
2019	27	[1]
2020	39	[1]
2021	45	[1]
Thereafter	58	[1]
Total	255	[1]
Nuclear Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	19
2018	67
2019	27
2020	39
2021	45
Thereafter	58
Total	255
Purchased Power
Long-term Purchase Commitment [Line Items]
2017	59	[1],[3],[4]
2018	170	[1],[3],[4]
2019	63	[1],[3],[4]
2020	13	[1],[3],[4]
2021	2	[1],[3],[4]
Thereafter	20	[1],[3],[4]
Total	327	[1],[3],[4]
Purchased Power | Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	0	[3],[4]
2018	0	[3],[4]
2019	0	[3],[4]
2020	0	[3],[4]
2021	0	[3],[4]
Thereafter	0	[3],[4]
Total	0	[3],[4]
Purchased Power | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2017	59	[3],[4]
2018	170	[3],[4]
2019	63	[3],[4]
2020	13	[3],[4]
2021	2	[3],[4]
Thereafter	20	[3],[4]
Total	327	[3],[4]
Methane Gas
Long-term Purchase Commitment [Line Items]
2017	1	[1]
2018	4	[1]
2019	4	[1]
2020	5	[1]
2021	5	[1]
Thereafter	64	[1]
Total	83	[1]
Methane Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	1
2018	4
2019	4
2020	5
2021	5
Thereafter	64
Total	83
Other Purchased Fuel
Long-term Purchase Commitment [Line Items]
2017	33	[1]
2018	57	[1]
2019	39	[1]
2020	39	[1]
2021	26	[1]
Thereafter	123	[1]
Total	317	[1]
Other Purchased Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2017	20
2018	44
2019	25
2020	25
2021	26
Thereafter	100
Total	240
Other Purchased Fuel | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2017	13
2018	13
2019	14
2020	14
2021	0
Thereafter	0
Total	$ 54
April 2017 Procurement | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	4,249,800
Long-term Purchase Commitment, Amount	$ 128
September 2017 Procurement | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	1,950,200
Long-term Purchase Commitment, Amount	$ 57

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] (a)Includes amounts for generation and for distribution.

[3] (b)The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2032 with various renewable energy suppliers due to the contingent nature of the payment amounts.

[4] (c)The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts.