Income Taxes (Reconciliation Of The Change In The Liability For Interest On Unrecognized Tax Benefits) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Income Tax Contingency [Line Items]
Liability for interest	$ 6	[1]	$ 5	[1]	$ 17	[1]
Interest charges (income)	(5)	[1]	1	[1]	(11)	[1]
Interest payment					(1)	[1]
Liability for interest	1	[1]	6	[1]	5	[1]
Union Electric Company
Income Tax Contingency [Line Items]
Liability for interest	8		6		10
Interest charges (income)	(8)		2		(3)
Interest payment					(1)
Liability for interest	0		8		6
Ameren Illinois Company
Income Tax Contingency [Line Items]
Liability for interest	1		1		2
Interest charges (income)	(1)		0		(1)
Interest payment					0
Liability for interest	0		1		1
Other Affiliates [Member]
Income Tax Contingency [Line Items]
Liability for interest	(3)		(2)		5
Interest charges (income)	4		(1)		(7)
Interest payment					0
Liability for interest	$ 1		$ (3)		$ (2)

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