Callaway Energy Center (Narrative) (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) mill	Sep. 30, 2016 USD ($)
Nuclear Waste Matters [Line Items]
Litigation Settlement, Amount			$ 24
Nuclear Plant
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee | mill		1
Annual decommissioning costs included in costs of service		$ 7
Subsequent Event
Nuclear Waste Matters [Line Items]
Litigation Settlement, Amount	$ 3