Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.2	0.4	0.5	0.7
Issuance of common stock	$ 18,000,000		$ 37,000,000	$ 40,000,000
Stock Issued During Period, Shares, Other	0.0	0.0	0.8	0.7
Noncash financing activity - Issuance of common stock for stock-based compensation			$ 54,000,000	$ 35,000,000
Union Electric Company | Secured Debt | First Mortgage Bonds, 3.50%, Due 2029 - $450 Issuance
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 450,000,000		$ 450,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Proceeds from Issuance of Secured Debt			$ 447,000,000
Union Electric Company | Secured Debt | Senior Secured Notes 6.70% Due 2019
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.70%		6.70%
Repayments of other long-term debt			$ 329,000,000