Long-Term Debt Long-Term Debt Fair Value (Details) (USD $) In Billions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract]
Long-term Debt	$ 3.0	$ 3.5
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Kansas City Power and Light Company [Member]
Fair value of financial instruments [Abstract]
Long-term Debt	1.9	1.9
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract]
Long-term debt fair value	3.5	3.9
Estimate of Fair Value, Fair Value Disclosure [Member] | Kansas City Power and Light Company [Member]
Fair value of financial instruments [Abstract]
Long-term debt fair value	$ 2.2	$ 2.2