Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Current Federal taxes	$ 5	$ (1)	$ (2)
Current State taxes	32	(3)	(4)
Deferred Federal Income Tax Expense (Benefit)	511	299	299
Deferred State taxes	34	92	76
Deferred investment tax credits, amortization	(6)	(5)	(6)
Total income tax expense	576	382	363
Union Electric Company
Income Taxes [Line Items]
Current Federal taxes	149	31	110
Current State taxes	23	6	17
Deferred Federal Income Tax Expense (Benefit)	76	161	71
Deferred State taxes	11	23	16
Deferred investment tax credits, amortization	(5)	(5)	(5)
Total income tax expense	254	216	209
Ameren Illinois Company
Income Taxes [Line Items]
Current Federal taxes	(34)	(8)	(83)
Current State taxes	29	12	(11)
Deferred Federal Income Tax Expense (Benefit)	185	117	193
Deferred State taxes	(13)	37	29
Deferred investment tax credits, amortization	(1)	0	(1)
Total income tax expense	166	158	127
Other
Income Taxes [Line Items]
Current Federal taxes	(110)	(24)	(29)
Current State taxes	(20)	(21)	(10)
Deferred Federal Income Tax Expense (Benefit)	250	21	35
Deferred State taxes	36	32	31
Deferred investment tax credits, amortization	0	0	0
Total income tax expense	$ 156	$ 8	$ 27