Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
State corporate income tax rate	4.00%	5.00%	4.00%
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 18
Union Electric Company
Income Taxes [Line Items]
State corporate income tax rate	3.00%	3.00%	3.00%
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 18
State
Income Taxes [Line Items]
State corporate income tax rate		7.75%
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		$ 53	$ 2
State | Minimum
Income Taxes [Line Items]
State corporate income tax rate	7.30%
State | Maximum
Income Taxes [Line Items]
State corporate income tax rate			9.50%
2007 through 2011 tax year [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 20
2007 through 2011 tax year [Member] | Union Electric Company
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 13