Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Income Taxes [Line Items]
Current Federal taxes	$ 31	[1]	$ (27)	[1]	$ 13	[1]
Current State taxes	3	[1]	(5)	[1]	10	[1]
Deferred Federal taxes	(590)	[1]	273	[1]	274	[1]
Deferred State taxes	(117)	[1]	76	[1]	36	[1]
Deferred investment tax credits, amortization	(7)	[1]	(7)	[1]	(8)	[1]
Total income tax expense	(680)	[1]	310	[1]	325	[1]
Union Electric Company [Member]
Income Taxes [Line Items]
Current Federal taxes	(25)		3		(14)
Current State taxes	(10)		2		(15)
Deferred Federal taxes	248		129		206
Deferred State taxes	44		31		27
Deferred investment tax credits, amortization	(5)		(4)		(5)
Total income tax expense	252		161		199
Ameren Illinois Company [Member]
Income Taxes [Line Items]
Current Federal taxes	(7)		(24)		(20)
Current State taxes	(3)		(4)		(5)
Deferred Federal taxes	76		123		132
Deferred State taxes	30		34		32
Deferred investment tax credits, amortization	(2)		(2)		(2)
Total income tax expense	$ 94		$ 127		$ 137

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