Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts receivable - trade, allowance for doubtful accounts	$ 50	$ 17
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000.0	400,000,000.0
Common Stock, Shares, Outstanding	253,300,000	246,200,000
Union Electric Company
Accounts receivable - trade, allowance for doubtful accounts	$ 16	$ 7
Common Stock, No Par Value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized	150,000,000.0	150,000,000.0
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade, allowance for doubtful accounts	$ 34	$ 10
Common Stock, No Par Value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized	45,000,000.0	45,000,000.0
Common Stock, Shares, Outstanding	25,500,000	25,500,000