Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.3	0.3
Issuance of common stock	$ 19,000,000	$ 17,000,000
Stock Issued During Period, Shares, Other	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]
Total principal amount of notes	$ 450,000,000
State interest rate	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]
State interest rate	6.70%
Repayments of other long-term debt	$ 329,000,000