Income Taxes (Reconciliation Of The Change In The Liability For Interest On Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
Liability for interest	$ 0	$ 1	$ 6
Interest charges (income)		(1)	(5)
Liability for interest	0	0	1
Union Electric Company
Income Tax Contingency [Line Items]
Liability for interest	0	0	8
Interest charges (income)	0	0	(8)
Liability for interest	0	0	0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
Ameren Illinois Company
Income Tax Contingency [Line Items]
Liability for interest	0	0	1
Interest charges (income)		0	(1)
Liability for interest	0	0	0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
Other
Income Tax Contingency [Line Items]
Liability for interest	0	1	(3)
Interest charges (income)		(1)	4
Liability for interest	$ 0	0	$ 1
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
2007 through 2011 tax year | Ameren Corporation [Member]
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		20
2007 through 2011 tax year | Union Electric Company
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		13
Tax Year 2012 [Member] | Union Electric Company
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		$ 18