Callaway Energy Center Callaway Energy Center (Cost and Fair Values of Investments In Debt Securities in Nuclear Decommissioning Trust Fund According to Contractual Maturities) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Nuclear Waste Matters [Abstract]
Cost, Less than 5 years	$ 120
Cost, 5 years to 10 years	54
Cost, Due after 10 years	54
Cost, Total	228
Fair Value, Less than 5 years	120
Fair Value, 5 years to 10 years	55
Fair Value, Due after 10 years	57
Fair Value, Total	$ 232