Callaway Energy Center 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 396	$ 377	$ 349
Gross realized gains	13	7	8
Gross realized losses	5	4	2
Union Electric Company
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 396	$ 377	$ 349
Minimum | Nuclear Decommissioning Trust Fund [Member]
Nuclear Waste Matters [Line Items]
Trust fund investments, target allocation percentage	60.00%
Maximum | Nuclear Decommissioning Trust Fund [Member]
Nuclear Waste Matters [Line Items]
Trust fund investments, target allocation percentage	70.00%