Callaway Energy Center (Proceeds From The Sale Of Investments And Related Gross Realized Gains And Losses In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 260	$ 299	$ 305
Union Electric Company
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	260	299	305
Gross realized gains	10	18	13
Gross realized losses	$ 2	$ 5	$ 5
Minimum | Nuclear Decommissioning Trust Fund | Union Electric Company
Nuclear Waste Matters [Line Items]
Trust fund investments, target allocation percentage	60.00%
Maximum | Nuclear Decommissioning Trust Fund | Union Electric Company
Nuclear Waste Matters [Line Items]
Trust fund investments, target allocation percentage	70.00%