Commitments And Contingencies (Schedule Of Lease Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	$ 32
Capital lease payments, due in two years	32
Capital lease payments, due in three years	32
Capital lease payments, due in four years	33
Capital lease payments, due in five years	32
Capital lease payments, After 5 Years	264
Capital lease payments, Total	425
Less Amount representing interest, due in one year	26
Less Amount representing interest, due in two years	25
Less Amount representing interest, due in three years	25
Less Amount representing interest, due in four years	25
Less Amount representing interest, due in 5 years	24
Less Amount representing interest, After 5 Years	24
Less Amount representing interest, Total	149
Present value of minimum capital lease payments, due in one year	6
Present value of minimum capital lease payments, due in two years	7
Present value of minimum capital lease payments, due in three years	7
Present value of minimum capital lease payments, due in four years	8
Present value of minimum capital lease payments, due in five years	8
Present value of minimum capital lease payments, After 5 Years	240
Present value of minimum capital lease payments, Total	276
Operating leases, due in one year	10
Operating leases, due in two years	9
Operating leases, due in three years	8
Operating leases, due in four years	6
Operating leases, due in five years	6
Operating leases, After 5 Years	14
Operating leases, Total	53
Total lease obligations, due in one year	16
Total lease obligations, due in two years	16
Total lease obligations, due in three years	15
Total lease obligations, due in four years	14
Total lease obligations, due in five years	14
Total lease obligations, After 5 Years	254
Total lease obligations, Total	329
Total rental expense	11	$ 38	$ 36
Union Electric Company
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	32
Capital lease payments, due in two years	32
Capital lease payments, due in three years	32
Capital lease payments, due in four years	33
Capital lease payments, due in five years	32
Capital lease payments, After 5 Years	264
Capital lease payments, Total	425
Less Amount representing interest, due in one year	26
Less Amount representing interest, due in two years	25
Less Amount representing interest, due in three years	25
Less Amount representing interest, due in four years	25
Less Amount representing interest, due in 5 years	24
Less Amount representing interest, After 5 Years	24
Less Amount representing interest, Total	149
Present value of minimum capital lease payments, due in one year	6
Present value of minimum capital lease payments, due in two years	7
Present value of minimum capital lease payments, due in three years	7
Present value of minimum capital lease payments, due in four years	8
Present value of minimum capital lease payments, due in five years	8
Present value of minimum capital lease payments, After 5 Years	240
Present value of minimum capital lease payments, Total	276
Operating leases, due in one year	8
Operating leases, due in two years	8
Operating leases, due in three years	7
Operating leases, due in four years	6
Operating leases, due in five years	6
Operating leases, After 5 Years	14
Operating leases, Total	49
Total lease obligations, due in one year	14
Total lease obligations, due in two years	15
Total lease obligations, due in three years	14
Total lease obligations, due in four years	14
Total lease obligations, due in five years	14
Total lease obligations, After 5 Years	254
Total lease obligations, Total	325
Total rental expense	10	34	34
Ameren Illinois Company
Commitments and Contingencies [Line Items]
Operating leases, due in one year	1
Operating leases, due in two years	1
Operating leases, due in three years	1
Operating leases, due in four years	1
Operating leases, due in five years	1
Operating leases, After 5 Years	1
Operating leases, Total	2
Total rental expense	$ 1	$ 30	$ 28